#1 Mentality
Mourinho has a reputation for being arrogant. In press conferences, he rides on the back of his past accomplishments. Yes, they may be great but does nothing to help the club's current fortunes.
He fails to inspire belief in his players and instead criticizes them openly. What does the open criticism do to a player's confidence? He has not shied away from taking a swing at the United leadership in the past which is not good for the club.
The United mentality should be of winning and positive play. Mourinho does not instil that in his players and would deploy the route one football of target men in the box which is speculative. He may have stated that this is his dream job but not all things work out so it might be time for the board to cut their losses and look elsewhere.