"I imagine that can happen to any online community. Eventually the polite, reasonable folks stop feeling like it’s a group of people they want to hang around. So they leave, and those who remain start to see only those who agree with them— and, because that’s all they see, they think that’s all there is. Everyone feels as they do, according to them. Once the tipping point is passed, you’re left with the extremes… those who hate, and those who dislike the haters enough to endure the toxic atmosphere to try and combat them. Each clash between those groups drives more of the others away."
It's been years since I've played WoW, but so far as I know their boards are also still highly toxic environments. The Battle.net community was famous for being awful even before WoW.
I know that even ArenaNet didn't want to host an official forum out of similar concerns that it would be too difficult to keep it from becoming an echo chamber of negativity and hostility. What is it about gaming communities that allow us to treat such awful communities where people dump inordinate amounts of vitriol on the devs and each other as normal and par for the course? It's been said that only a tiny percentage of a game's playerbase ever sets foot in a game's official forum (or any forum), so at least we can believe that it's just a minority responsible for the majority of this problem, but it still doesn't remove the bad taste.
Related Reading: Taming the Forum Tiger