The refs hate us because we ruined their childhoods.
That may well be the case for some but I think it's often also more subtle than that.
If United get the benefit of a decision the issue is highlighted, analysed and discussed in the media endlessly (Onana against Wolves for example). Referees are criticised, apologies are called for and even granted in some cases.
On the other hand if we are the victims of a dubious decision the matter is skirted over and hardly mentioned (e.g. Hølund being flung to the ground at Arsenal and several others this season alone). The officials don't get called out.
I think this pattern creates unconscious bias in the minds of officials and in the split second when decisions are being made it results in a risk-averse response.
There's also the narrative that "United always get the decisions". This canard may lead referees to try to prove they are impartial by going to the other extremes (witness how often and how easily our players get yellow cards compared to other sides, for example.)
I suspect Gary Neville on Sky commentary suffers from this one. In an effort to avoid accusations of bias he seems chronically incapable of calling any 50:50 decision in United's favour.