Just a guess, but it might be being injured and henceforth his patchy involvement with the team, and subsequent poor results.
It's not rocket science.
It doesn't mean he's plotting an escape or leading a revolt. It just means he wants to be fit, playing, scoring and winning. He never looks unhappy when this is the case, as you know from when we beat you.
If he can get back fit and scoring, with United winning matches as a result, all this daft bollocks will cease again... until we lose a game and he plays poorly, and shock horror, doesn't look happy, then we can go back to talking about how he wants to smother Moyes in his sleep and sign for Liverpool.