It's not a question of criminal behavior but the bank's interpretation of the law versus the government's. As long as banks don't draw attention to themselves and don't get sued, they can operate largely in gray areas of the law to get and keep clients, make a little extra profit, take a little extra risk, etc. Once they are getting audited with a fined toothed comb, anything that comes up where the bank's interpretation is at odds with the government's can be easily found out and people can get punished.
This can be anything from how the bank values its assets (which... who cares, they're going bankrupt anyway, right?) to director malfeasance or negligence which can result in jail time. In normal circumstances almost all businesses operate this way at least a little bit, but in banking there's a paper trail and when it's under a regulator's microscope, it's much more likely to get noticed and punished.