It might not just be identifying with something. It doesn't seem that it's completely identify based, at least to me.
I drive a Jeep -- love old Jeeps. It's part of who I am. But if somebody started ragging on Jeeps in a forum I was in I don't think it'd matter. I wouldn't feel threatened. I like Jeeps. So what. I also like puppies.
Some subjects make people feel threatened because to talk about them, in any fashion, is to personally criticize their judgment and their intelligence.
It's the degree people feel threatened. Most people, when you poke around at their politics or their religion, realize their is something deeply irrational going on in their head. They don't take kindly to having people mucking around in there, questioning their sanity -- their value as a fellow person. Religion and politics are topics where the fact that emotions drive reasoning is very close to the surface. It makes probing difficult.
As a further example, I disagree with Paul's opinion on languages. Inevitably we get into a discussion of what "best" means. At the end of the day, "best" usually means whatever I personally value. To criticize that is to criticize how I make opinions. Paul values terseness. Bob values availability of programmer. Joe values available libraries, etc. Once we start questioning judgment -- especially for programmers -- it gets personal.
I've been in the consulting biz for a while -- long enough to move from code monkey to trying to help managers make better decisions. A lot of this is like poking around in a minefield: on some topics people feel their character/intelligence/judgment is a lot more at stake than others. These are the topics you don't want to go bringing up in a forum, online or not.