There are many factors at work in this. I will simply draw on a few, and others can go from there.
If I start a thread with a title …“All Catholics worship statues and are going to hell!!”, that would obviously be seen by Catholics as “anti”, and any conversation would obviously go down hill from the first post. So, calling that original poster “anti-Catholic” would be justified.
There is also the issue of “tone”. It is very difficult typing a statement and getting the “tone” of what you’re saying through to the general reader. For example, you could read a statement and be offended, while I could read the same statement, and see nothing offensive. It is a matter of perspective.
Then we get into posting history. Posters on these threads, including myself will have encounters with other posters on different threads. In many cases, that history carries over into the current conversation. It isn’t like we’re meeting each other for the first time on each thread we post on…
As far as the term “anti–insert religion here”, it is thrown around alot. Maybe too much, but that is hard to say. You need to remember that most of the people on here are not professional apologists. They don’t have the answer to every question, no matter how much they would like to believe they do.
You mentioned that you see this when you think a poster has been backed into a corner so to speak. When he/she cannot defend a point.
What you will actually see in most cases if you look through an entire thread is that the person who cannot defend their position will abandon the thread (quit posting on it)
Just a couple of my observations, I’m sure you’ll get more from others.