I don't know how old you are, but the anti England feeling among many United fans goes back a long way and it has nothing to do with them being crap.
For many it goes back to the shocking abuse Beckham received from Ingurland fans. He was booed at nearly every ground for months after '98. He's far from alone too. Both the Neville brothers got pelters for mistakes playing for their country, for example. Stand up if you hate Man Utd rang around many England games in the 90s as well.
Also it's the racism. The awful behaviour abroad. The disgusting xenophobic songs from the fans.
The press and media don't help either. The constant cycle of blind optimism and chest beating. Ignoring obvious shortcomings and not allowing any room for realistic appraisal, followed by scapegoating and blame when they inevitably don't win, making the whole experience of watching England really quite painful.