It's quite simple - Everton doing well undermines his reputation, and thus hurts his standing at United at a critical time.
Moyes built up his reputation, fueled by the national media who were constantly looking for 'The Successor to Fergie' (and were being briefed constantly by Utd sources for years that Moyes was 'the one') that everything Everton achieved was down to him and him alone and that the club would completely fall apart without him.
The club hasn't, it has gained a new lease of life under a far more positive manager, who plays a better brand of football than Moyes, and who signed 4 players in the current side, and plays a 5th (Barkley) whom Moyes refused to play.
That's bound to result in the Utd hierarchy and fans looking at Moyes and wondering if they got it wrong - if he really was as crucial to Everton as was made out. Lets get this straight - we don't have the same team we had under Moyes last season. We have up to 5 new players in the side (nearly half the team) and we play a completely different system.
Moyes is clearly a little bitter. Think of it from his position - he thought he was the dogs bollocks, yet as soon as he leaves, everyone raves about Martinez and how things are so much better. His ego has bound to have taken a knock.
