Anonymity is one aspect, for sure. But also, people often have not taken in the lesson of interpreting generously. They take the worst interpretation of the words, and often add a few imagined things that were never written (oh you like healthcare, you communist?). This is worse in written form because the feedback loop is longer, but it being written makes it feel like the author has had plenty of time to think about it.

Forums are also the kind of place that everyone thinks are populated by political bots. Believing every other comment is written in bad faith is going to change how you behave.

When you think of arguments as a kind of battle, you end up forgetting the person on the other side.