I call my theory on reddit sub moderation the 'dead sea effect'.
If your sub exists as a sort of support sub for one small group that feel they are oppressed by $largerGroup, the most bitter, jaded and cynical users will be the most active. The most active users are the most likely to be recruited as moderators. Those moderators then do not bother to moderate content that matches their more extreme world views, and that pushes more reasonable folks away from the sub. Before you know it, the sub's tone turns more bitter, the wheel turns, more moderate folks leave, and on it goes till your sub is now a profoundly negative and bitter space.
It takes very proactive moderation to prevent that slide, and most folks aren't equipped for it.
If your sub exists as a sort of support sub for one small group that feel they are oppressed by $largerGroup, the most bitter, jaded and cynical users will be the most active. The most active users are the most likely to be recruited as moderators. Those moderators then do not bother to moderate content that matches their more extreme world views, and that pushes more reasonable folks away from the sub. Before you know it, the sub's tone turns more bitter, the wheel turns, more moderate folks leave, and on it goes till your sub is now a profoundly negative and bitter space.
It takes very proactive moderation to prevent that slide, and most folks aren't equipped for it.