I think with United it often comes down to the "Beaten before they start" : "Raise their game" ratio as far as their opponents are concerned, particularly at Old Trafford.
Under Ferguson that ratio was about 3:1 which meant that they often picked up points when not playing well. It was a huge factor in 11/12 and 12/13. Under Moyes and Van Gaal the ratio declined and then to some extent reversed. More teams actually enjoyed facing United, sensing that they could beat them.
Under Mourinho the ratio has started to revert back towards that of the Ferguson years. Its fueled by the hugely favourable media coverage they, and Mourinho, receive whenever they have a few good results. They're made out to be much better than they actually are which eventually has an affect on the mindset of their opponents.