This is pretty understandable behavior, and was basically inevitable once they reached a certain userbase size. Its human instinct to only care about inclusion when you feel like new blood is a resource, and to care more about enforcement and exclusion and power tripping when new blood feels more like competition. And that happens at certain threshold population sizes, regardless of how "nice" people think they are before their instincts get switched on.
Basically, the seeds of this whole mess may look cultural, but the actual behavior complex kicks in whenever the total "mod clique" exceeds Dunbar's number - which it did around 2006 or so.
You can see the same pattern play out on somethingawful, or any other big board you might care to look at.
Humans simply cant handle systems this big, with emotional feedback loops that are this tight, without going absolutely apeshit.
The "good" boards embrace this, and fold the apeshittery directly into their culture. But they still look just as horrific to outsiders.