I think there are several types of flounce, having observed a fair few in my time on SH.
1. The 'you're all bastards and I hate you; I'm never ever going to frequent this forum again' This is often a reaction to something another forumite has said. It's the most dramatic one and is made more entertaining when the flouncer comes back. My personal favourite type of flounce, to watch.
2. The 'poor me, I'm going because I can't get any meets/PM's' flounce. A clear attempt to garner sympathy and a drift of PM's /offers of NSA sex.
3. The sneaky flounce - where they don't start their own flounce thread; they kind of post something in another thread about their boredom/annoyance about the forum/site, and how they're going to not bother any more.
4. The 'nobody takes any notice of anything I say so I'm going' flounce. A clear attempt to get forumites begging the flouncer to stay because they hang on the flouncer's every word.
Anyone seen any different ones, that I've missed?