I don't have anything specific to say about the subreddit. I never visited it and have no experience. However, I do find it a sad fact about human nature that we can bond so strongly around opposition to another group of people, which can be defined in almost any arbitrary way, as long as a commonality of negative opinion can be reached.
All human bonding is based on an arbitrarily common opinion. I highly doubt you have any friends who you disagree on everything with after all. I personally have most of my friends and bonds come from the queer community in my city. It's completely arbitrary and says little about our personality, but it's a commonality we could bond because of. We share the opinion of anti-queer people being bad people, and that's why we bonded. Not only can we strongly bond around opposition, we are required to bond around opposition. What I'm saying now is all conjecture and not on any level of research, but if you think about pre-historic days, it makes sense that we would all bond when a problem arises, that way we as a group can fix the problem, and it's a boost to morale knowing others have the same opinion. It just so happens that the biggest threat to today's society is arguably the obesity epidemic which is overtaking smoking in it's death toll. If instead, say, windows were a huge issue in society killing more and more people every year, chances are I'd be over on /r/glasspanehate or some similarly named subreddit. That being said, I cannot hide the fact that /r/fatpeoplehate was a hate subreddit. It had people there specifically to stir up shit. There were people who were overtly hateful, brigading people, wishing them death on their social media, etc. I am not proud of this, nor did I or any of the mods condone this behaviour. It was however the exception, not the rule.