Why do teams keep doing the same garbage at the end of games against Man Utd?

Every time they have a lead or are on course to gain a point from them, as soon as added time is displayed they fall back into defence mode, allowing man utd to bring even van der sar into their box to take shots on goal. What the fk! I see this too much and "Fergie Time" jokes aside, this is always the reason why Utd scores after the 90th minute, cause they are encouraged to.