Some observations:
1. People hate seeing false 'facts'. Someone often makes an entirely stupid claim and someone will counter that with another stupid claim. E.g. "It's scientifically proven that Asian genes are inferior. This is common sense." "No, here's an entirely unreliable blog post that proves that Asians are indeed the master race." "If Asians are the master race, then why are their birth rates so low? Everyone knows that high fertility is the foothold in any empire."
That's part of why political/religious debates go so long. The more people involved, the higher the likelihood of stupid claims.
2. In real life, when people start arguing, they get tired after a while and say sorry. In the internet, especially forums, once they get tired, someone covers for them.
3. Anonymity can be quite powerful. I put up less controversial stuff on my Facebook/LinkedIn, and nobody argues on it. Heck, I do it just to start arguments/discussions. I'm guessing it's because the photo makes them think twice about it, or that they're not willing to argue certain things in front of certain people. FB/LI is also a bitch when it comes to anonymity, argue with a stranger via Facebook, and it literally tells your best friends and spouse. Personally, I don't mind people finding things I've talked about with other internet strangers, but a lot of people seem to show different faces to different people.
4. Some people get into the habit of throwing punches on certain triggers. Like mentioning 'creationism/evolution/Muhammad' will trigger a fight.
5. It takes restraint to prevent a controversial thing from snowballing. Once one person fails to restrain themselves, it lowers the willpower of others as well.
6. Some people eventually get frustrated by either talking with idiots (in their opinion) or being treated like an idiot and resorts to trolling. It's a defense to prevent themselves from being seen as legit idiots. It's a way to reduce the pain of getting an emotional response from a perceived idiot... by actually trying to get an emotional response, they'll enjoy it.