Classic Eternal September.
Around 60k subscribers, the cultural identity starts degrading, as the amount of "old guard" is outmatched by "new blood." Therefore, the old "monkey see, monkey do" phenomenon, where new users would slowly mimic the culture of the prevailing older users to "fit in," is replaced with new users mimicking other new users, and the culture shifting towards the platform's identity instead of retaining the sub's identity.
Generally, the type of person to post on Reddit frequently enough, has social cohesion problems that may preclude him from fulfilling his social needs through more healthy avenues, like real life. The same is true for the majority of people that post online frequently. Usually the pyschological profile that follows that point is one built on abrasiveness, distrust and aggression towards authority, an inability to adopt social manners and participate in social contract, low emotional intelligence, etc.
All of the aforementioned behaviors culminate into the toxicity and vitriol you usually see -- and as well why it's so prevalent.