It's a long, long story. The short version is that about 23 years ago, one of the projects I was working on was being covered in The Oregonian. At that time, their website had forums for people to argue and converse, and one of them was spreading misinformation about our work. So my supervisor told me to go in and correct it. Someone at that forum saw that I was a biologist and invited me to the religion section to counter the creationist stuff being posted there. I did and found I liked it, so over the next 15+ years I sought out places to debate with creationists.
But in the last ~5 years the evolution v creationism debates have almost completely died, so I started debating current events, politics, etc. more, and I guess I'm still in the habit of doing so mostly at Christian forums. It works for me because I don't like going to places where I agree with the majority.
So I guess the even shorter version is....it's kind of a habit.