Agreed it obviously completely altered the course of the match - which is extremely unfortunate for us.
But I’m not one for conspiracy theories. It could have been an honest mistake. I was a lot further away from the incident than the ref was, but from the same angle, and my immediate reaction to GSMs challenge was that’s going to be a red card - before he brandished it. Mellon’s reaction immediately after the game was also that he wouldn’t be appealing it “because they never get overturned”. Well, obviously they do. Not often, but they do. Can only assume that it looked completely different from another (tv) angle.
More generally, when a (any) ref has, once or twice, given an important and controversial decision against a particular team - and inevitably been lambasted for it by that club’s manager and fans - he is likely to have a subconscious prejudice against that club’s manager, when reffing future matches involving them.

More often it’s against specific players than a club in general. Mike Fondop has a problem with ref’s at our level generally, because whether rightly or not, he has gained a reputation for violent conduct.

More generally, I think it works both ways. Back in the premier league days, I was always delighted when we got Roger Milford assigned to our matches. We always seemed to get the rub of the green on any controversial decisions when he was in charge.