What causes people to resign?

People always act in their own self interest.

CEOs resign to avoid the embarrassment of being forcibly removed by their boards of directors. Someone friendly from their board will contact them and say, We've heard such and such, going to need to investigate and think about removing. If the CEO knows it to be false or trivial, he will fight it; otherwise, resign.

Career government employees will almost never resign, at least not until Trump signed that order to make it easier to fire government employees. The exception is when they are being offered a high paying private sector job - usually something to do with lobbying or government relations. Otherwise, why resign?