I was about to write that it's always defence ministers that resign, probably because of money/priorities.
But then I realised the churn over the last decade plus (Fox, Hammond, Fallon, Williamson, Mordaunt, Wallace, Schapps) was for a variety of reasons (conflict of interests, promotion, sex scandal, leaked info, change of leader, NATO leadership hopeful, GE).
I'm sure its a difficult role to fulfil without the budget that they'd like to have. But I suppose the same could be said about multiple departments.