It isn't corruption. It isn't strictly incompetence, either, I think--at least not in the sense of them not knowing the rules of football or whatever. It's simply ABU influencing referees, pressuring them, launching witch hunts anytime they give us anything. It's the fact that a referee knows in the back of his mind that if he gives United anything, he gets hounded down in the media; but if he robs us, there are no consequences. They're subconsciously afraid of becoming the target of the heavily Liverpool-dominated football media in England, and they know deep down that if they give us anything at all, they will be. That makes them over-scrutinize everything United's players do, and it makes them interpret the actions of our opponents through a lens of leniency.
Look how long people harped on about the City goal, and that was a correct decision. Even when a ref makes the right call, if it happens to be good for United, they become the victims of an insane smear campaign and weeks of scrutiny, judgment and criticism. So they sabotage our games again and again because that's safe for them, it doesn't anger the hysterically irrational ABU movement, and nothing happens because we're not allowed to complain. We're not even allowed to acknowledge that this is happening without people laughing it off. The anti-United thing is very real and very strong.