Zeeman being second in command, changes that high in the power structure almost always have some deeper causes.
he wants to show he's better at training a football team than Brailsford
I dont know there could be loads of reasons, boredom, theyve offered him more money, its a more stable work environment, AZ arent about to go out of business every year if their main sponsor ups sticks, or it provides better work/life balance options for him.