I edited my comment for precision, but what happens is that the bank finally notice that you are not as eager as the next batch of freshly promoted coworker. (i.e. you have a life on the side, you cannot be contacted during the we, you go on holiday, ...)

So if your department made slightly less profit that year, you get selected in the next batch of firing that happen generally just before bonus time.

Note that firing is a regular event for bankers. There is not much stigma attached to it, you just need to have the willpower to use the opportunity to retire.