It's like when the Catholics, the Orthodox, and the Anglicans all get together and rag on Evangelical Protestants. Which is also something I really enjoyed participating in as a kid. (It's called sectarianism for a reason. We're worse than the churches sometimes.)

There's sectarianism, and then there's dunking on people the entire left agrees are just plain wrong and tend to be crank magnets.

At least for me, as a former trot, it's like 70% a joke. A lot of them are either cranks or NATO-leftists (or both) but a lot of them are also basically normal