We have an extremely aggressive site culture from our origins on Reddit when people who didn't agree with our general political tendencies would wander into our subreddit and act like obnoxious smuglords once they realized what we're about. Rather than outright ban these people, we would bully them until they left - and even though we have somewhat stricter moderation on this site than we did in the subreddit, the viciousness stayed. We also have an extremely online sense of humor, given that we were founded to discuss a left-wing twitter podcast, which can make the general tone dense and impenetrable for newcomers.

calling for acts of violence

FWIW the subreddit was actually banned because it said that John Brown killing slave owners was good which Reddit called "promoting violence."