While the BCB release didn't specify the nature of the breach, ESPNcricinfo understands that the incidents took place during the Chittagong leg of the ongoing season.
"A Governing Council investigation has found both players guilty of serious off-field disciplinary breaches during the current tournament," a BCB press release read. "As a consequence, Al Amin has been penalised 50% of his BPL contract fees while Shabbir has been fined 30% of his BPL contract amount.
Sabbir, placed in Grade A+ in the BPL player draft, was acquired by Rajshahi Kings for Tk 40 lakh (approximately USD 50,000), while Al Amin in Grade A was retained by Barisal Bulls for Tk 25 lakh (approximately USD 31,000).
"The players have been reminded of their responsibility as national cricketers and have been warned that any repeat of similar acts of indiscretion in the future will result in harsher penalty," the statement said.