I do not think games are fixed by the league or there is a general conspiracy for/against a specific team.
I think three things could occur:
1. Subconcious bias against a team.
2. Refs protecting themselves or other refs to provide cover/make points to certain players/teams.
3. As with a NBA ref several years ago, fixing by a single ref to make money. It was shown calls do not need to be obvious, just reasonably close.
I.suspect every fan things their team is the result of bad officiating. It would be an interesting study to see if they do average out.