Anonymity is a very large part of it. Most assholes on the internet are usually like that because there is very little chance that it could have serious repercussions in the physical world, which is the only thing they actually care about. If there is no consequences, then why not act horribly. I'm not saying that it's right, or that I agree with it, but I feel that it's the mindset that most people have.
Another interesting thing to consider is that it doesn't just work in one way. For instance, someone who is a jerk in real life might be nice on the nets (unlikely but possible).
A good way to look at the situation is to think of the internet as one immensely large MMORPG, but with a completely different basis for the games purpose. In a regular mmo, you start out with a blank slate and proceed to build your character to become as strong as is possible in the game to defeat monsters and such. In contrast, the purpose of the internet (game) is to become as known/recognized as possible(either good or bad) through likes on facebook, or brohoofs on this forum. If that wasn't the case, why do you think that people make endless Facebook/Twitter/Instagram posts vying for your attention, or try their hardest to gain fame as a troll.
On a forum it goes to a whole new level because of a much smaller and constant community in comparison to the vast internet. First you create an account on a given forum and that account starts out as a blank slate, with your actions from then on being what you are remembered by in the forums community. Whether you are a kind hearted person or a hardcore jerk/troll, you are going to be recognized as such. The latter tends to get recognized more often because most people have a tendency to focus on the bad in things, when in fact there is quite a bit of good occurring as well.
Whether any of that is even remotely understandable, I think it fits the topic well enough.