1. A forum thread grows when people have opinions on the topic, prompting them to post. It keeps growing if those people have opinions on each others opinions.
2. People posting opinions would like those opinions to receive attention of some kind or another, or they wouldn't post.
On a forum, a creative 'riff' can happen when people start making stuff in response to stuff made by others - giving us story-making forums. A debate can run along, garnering evidence and depth of argument on all sides (though it will almost never resolve, it may well be explained). An exploration of a topic can lead to experiences posted on all sides, with lots of people asking for more detail from one another. And, of course, a fight can steadily escalating into a match where everyone is throwing burning shit at each other.
These things all happen by means of the two points above; the mechanism is identical.
Now, on a large forum, this process can happen fairly swiftly. In a couple hours, a thread explodes, because there are lots of opinions. Notably, though, fighting can derail any debate, exploration, or creative discussion; if it gats hot, it's likely to stay hot - cool reaction in a fight takes energy. Which means that many of the biggest threads on any large forum are, by nature, going to be fights eventually, and stay that way.
Unless something intervenes, be it an authority, or the community at large.