Generally bank systems are very poorly built in a rush with exceptions to architectural advice all over, this is piled on top of older systems equally bad going back to some systems so old they beg staff not to retire and scour ebay for parts.

Bank maintenance is outsourced to the lowest bidder or the ones with the most hair gel.  The outsourcers can then only make a profit if there are faults.  The result is pretty obvious: the systems develop faults. 

It's a lot like jenga: push a bit here, pull a bit there and over it goes and no one quite knows why . (But the boss buys everyone beers and pizza when it happens 'cause it's just paid everyone's wages for the next few months  )