Frauds are very often hidden or disguised as "bad investments" (yeah banks do mistakes as any other human beings) and covered with money from "slush funds". When they are on vacations, rogue traders cannot cover their intentional bad investments, losses, close warning notifications and so on. More kn this topic here https://www.fdic.gov/news/financial-institution-letters/1995...

Two reasons: lots of fraud requires continual maintenance, and when you're away someone will presumably take over your duties and might notice what you're doing.