Andrew Miller is UK editor of ESPNcricinfo. @miller_cricket
Was it the fact that Ed Smith was too clever by half, and unafraid to show it? Or was it the backlash to England's rest-and-rotation policy - an imaginative response to the ongoing Covid crisis but one that proved too rigid for the already daunting challenge of facing India on home soil?
Or was it a simple act of cost-cutting from the ECB - a body that had to axe 62 jobs at the end of last summer due to a £100 million hole in its finances, and which can point to the pathways programme fronted by the ubiquitous Mo Bobat, as well as the over-sized squads for England's bio-secure tours, and claim with some legitimacy that the role of a bespoke selector really is surplus to current requirements?
Whatever the reasons (and given the depth and complexity that Smith liked to bring to his role, it's fair to assume they were myriad), his non-retention, or "dropping", to use the word that selectors themselves are so averse to uttering in this day and age, is a remarkable turn of events. It marks the end of more than 100 years of independent selection panels for England Test teams, and ushers in an unlikely new autocrat in the guise of head coach Chris Silverwood - or "Kim Jong Sil", to use the moniker that the Guardian's cricket correspondent doesn't expect to stick.
Rumblings abound about Smith's relationship with the players he was tasked with picking - many of them are understood to have been prickly in the extreme - and there was always an accompanying sense of over-complication to his methods. After all, once you've factored in a few elements of horses for courses, as well as the familiar vagaries of form, selection ultimately comes down to a fairly simple case of "yay or nay"? If this bloke cuts it, he's in for the duration; if this bloke doesn't, he's not. It's pretty mundane, actually, especially when the team is functioning well - which to Smith's credit, it was … at least until India cranked up the spin settings this winter.
But it's telling that, of the 28 players to feature in his 37 Test selections from May 2018 to March 2021, Smith's most constant "other-ranks" pick (behind the captain Joe Root) was a player who might never have played the format again, had he not backed his first and biggest hunch to the hilt.
Only a selector willing to approach the role differently - and willing to back up his leap of faith with a highly-evolved explanation - could have risked the recall of Jos Buttler for the Tests against Pakistan in 2018. Buttler had not played any Test cricket for 18 months, since a spare-part role on the tour of India, but Smith recognised that his rampant form for Rajasthan Royals in the IPL was there to be harnessed across formats, whether or not it offended any sensibilities in the process.
Sure enough, Buttler thrived on the faith that he had been shown, and translated that IPL confidence into a series of agenda-seizing displays : a player-of-the-match performance against Pakistan at Headingley, and an integral role in England's 4-1 series win over India, when two other Smith hunches, Sam Curran and Adil Rashid - back in contention despite quitting red-ball cricket for Yorkshire - helped provide the lower-order with enough depth and indomitability to wrestle an improbably comprehensive victory out of a tightly-contested series.
It's arguable, however, that Smith's vision was too all-encompassing for the remit of his role. When Andrew Strauss appointed him in the spring of 2018, his stated aim had been to usher in a new era of data-driven selection, and the early signs were undeniably fruitful in that regard. Yet his project stalled abruptly in the immediate aftermath of perhaps his finest hour.
England's 3-0 series win in Sri Lanka in November 2018 was a triumph for what was dubbed "total cricket" - the ability to be ultra-flexible and turn to players with specific skills in certain conditions, such as the subcontinent specialist Keaton Jennings or, less convincingly, Smith's former Kent team-mate Joe Denly, whose job-a-day legspin would be consistently over-sold in red- and white-ball cricket alike.
For three Tests against Sri Lanka, it worked a treat. In spite of fielding a team with more wicketkeepers than frontline fast bowlers, England's line-up had enough moving parts to cover every facet of the game: three contrasting spinners (leg, off and left-arm), a variety of pace options with Ben Stokes as the pivot, genuine batting to No. 8 and competence all the way down, and an enviable blend of ballast and flair therein. He'd cracked it within six months. It really was a simple game, especially for such a clever-clogs.
But unfortunately, Smith's ultra-logical treatment of players as chess pieces ran counter to the need to massage a few egos along the way, and also rode roughshod over the unspoken truth within dressing rooms - that not all players are equal. The decision to back Curran's ubiquity over Stuart Broad's single-string class in the subsequent Test against West Indies in Barbados in January 2019 backfired so spectacularly, amid a series-defining 381-run defeat, that it's arguable whether Smith ever quite had - or was permitted - the courage of his convictions again.
Certainly, he seemed to pull his punches in subsequent selections - perhaps most notably in the decision not to turn back to Jennings for this winter's subcontinent trip, even when Rory Burns dipped out of the Sri Lanka series to attend the birth of his first child. Likewise, we may never know now whether Dawid Malan - who took umbrage at Smith's early suggestion that his game was better suited to Southern Hemisphere venues - would have been a bolter for this winter's Ashes squad.
The equally plausible reason for Smith's belated conformity, of course, is that England had by that stage turned around their Test fortunes, thanks to the core of young players whom Smith himself had been instrumental in choosing: most notably, Ollie Pope (despite picking him at No. 4 on debut and, bizarrely, as a wicketkeeper in New Zealand), Zak Crawley, Curran and Dom Bess - yet another hunch pick, after Jack Leach broke his thumb on the eve of Smith's first squad selection. His temperament at Test level could not be questioned until his technique deserted him in India, and at the age of 23, he's got time to come again.
As for England's white-ball plans - the over-riding priority for 2019, his first full year in the role - Smith's involvement was never allowed to evolve beyond peripheral. His attempt to shoehorn Denly into England's World Cup plans was plain weird, and rightly kiboshed on the eve of the campaign, while his inability to offer any real hope of a recall to Alex Hales was the first true sign of Smith's redundancy, in the literal sense. What, honestly, could his purpose be, if Eoin Morgan had such a powerful and unchallengeable veto?
There were other mis-steps along the way - Jason Roy as an Ashes opener was the hunch that proved it's all guesswork really, while Smith's chastising of Jonny Bairstow for crimes against Test-match batting technique was inconsistent to say the least. Was he a wicketkeeper, was he a No. 3, was he both or was he neither? By the time he's sloped off the India tour with three ducks out of four, the latter seemed the likeliest answer. Something similar might also be said of Moeen Ali, who featured in just 11 of the Tests of Smith's era, despite for a 12-month run - up to and including his axing after the first Ashes Test at Edgbaston - being the leading wicket-taker in the world.
Despite all this, Smith finishes his stint in credit. England were drifting as a Test team when he took the job in the penultimate year of Trevor Bayliss' reign - they'd been trounced in the Ashes and bowled out for 58 against New Zealand, and were in the midst of an abject overseas run of 11 defeats and two draws in their previous 13 overseas Tests. Until the recent denouement in India, that record had briefly been transformed to six away wins in a row, but overall, a return of 21 wins and 12 losses in 37 Tests shows clear progress.
From a stodgy start, England found themselves challenging for the World Test Championship final right up until the penultimate Test of the cycle - and, for all that the knives were out after the apparent scuppering of the India tour, Smith's unapologetic adherence to England's rest-and-rotation policy has set a course for Covid survival that may yet have more spin-off benefits than are being appreciated right now. Dale Steyn's tweet said it best: England are creating an "army of amazing cricketers", with the depth of options necessary to survive a horrific and never-ending itinerary.
And if one or two England players are seething about the way they've been treated along the way, then it just so happens that professional sport provides a very productive outlet for such emotions. As Broad demonstrated in the wake of his snub at the Ageas Bowl at the start of the 2020 summer, sometimes it helps to have a voodoo doll in your hold-all to help channel that rage.
When, in the coming months, England's players find their style cramped within the dressing room, and have to bite their tongues for fear of getting on the wrong side of the captain-coach combination that now has a more official hold over their careers, they may yet have reason to miss Smith's defiantly independent line of thought.