Too much of anything happens on Twitter. People evolved to resolve conflicts face-to-face. It's so easy to interact on Twitter in a way that very few ever would in "real" life. It doesn't feel good to be mean to someone's face, especially if they seem like a mostly nice and reasonable person (as most are). Take that away and we naturally fall into toxic shout matches that go nowhere and slowly emit a cloud of intangible tension and stress that hangs over everything.