This happens to loads of teams, I don't think it's unique to us. It's also happened to us in the recent past, under OGS, when we had the record breaking away run but could barely win at home.
Personally, I don't think it's only nerves, I think it's also expectation. If you play Brighton away, the United fans will be singing for 90-mins, regardless of the scoreline, and mentally, the players will be thinking "a point isn't bad here" (whether they should be or not).
At OT, the expectation is that they win, and win well. The stadium is usually pretty quiet for middling home games and so every misplaced pass or poor effort on goal is met with a loud grown. There's also that anticipation as every United fan stands as we attack the opponents goal, and the disappointment when the move invariably breaks down.
We have a very young XI and a bunch of new players settling in, plus confidence must be at an all time low.