That's the trick, isn't it, though? Rationally it's especially a problem if it happens all at once, but realistically if your bank has an awful quarter then you're at fault whereas if every bank has an awful quarter then no one's at fault, it's a systemic issue, chances are you get to collect a bail out and keep going. It's a sort of prisoner's dilemma, a critical mass of banks needs to defect and each individual bank's incentive is to cooperate.