The main reason if probably because of Rooney, is it not?
I'm a casual watcher of soccer, so I probably wouldn't know hahah.
Probably a lot more to it than that... although he isn't the most popular player in the country so he doesn't help matters haha
The fans are stereotyped as being arrogant and many people hate Sir Alex Ferguson, the manager/coach, and he's accused of using his status for getting away with anything, really, and just generally doing what he wants. And they're accused of using their status to get away with doing what they want on the pitch - people consider that a lot of referees are too intimidated by Manchester United to actually make a decision against them. Also, a lot of people hate Manchester United because the majority of their fans aren't even from Manchester. They did a map and pin pointed the location of the average Manchester United fan based on the addresses in the official Manchester United Fan Club and it was somewhere in the middle of the Atlantic.
Fortunately for me, though, I am from Manchester