The reason for this is the same that applies to this ‘Fergie time’ nonsense. Firstly, a bad refereeing decision at OT is magnified in the media who then nurture the controversy and whose coverage also mean that we seem to think that it happens very regularly. But, this kind of thing happens all over the country during a weekend of football! Just because Man Utd are possibly the most hated club in England, the controversy is much bigger. So, if once or twice, a goal has been scored after injury time has elapsed (keep in mind that when it says 4 minutes, it can be 4 minutes and 1 second or 4 minutes and 59 seconds), people start to get paranoid and start to invent new theories as to why Man Utd keep challenging at the top of English football.
I have also noticed that whenever teams are sensing a good result at OT, the amount of time-wasting that happens is extraordinary! Against Wolves, Hahnemann was taking an eternity for each goal-kick. Because, for a lot of teams, even a draw at OT is a good result, all kinds of tactics are used to delay the normal flow of the game as mucha s possible. Then, when the referee puts up 6 minutes of injury time to be played and actually plays 6 and a half minute or more, every non-United fan is in upheaval about how the whole system has been corrupted by the dark and evil Man Utd!