I think Mourinho feels under pressure after a dismal season in the PL and a poor performance in the FA Cup final.
The Man U supporters are getting to him because they see their two biggest rivals, Liverpool and Man City, with exciting managers playing very attractive football and getting success, while Mourinho plays boring football because it’s the only way he knows.
He got away with it at Chelsea because they were a nothing club, going nowhere, and then they started winning the top competitions, and their supporters couldn’t care less if they their style was boring, as long as it wads effective.
At Man U, the expectation is, not only to win trophies, but to do it with style and attacking play, and he has done neither.
The only amazing thing about this situation is that they gave him a new contract.