It's not 'some people' leaving loads of negative rep though, is it?
You can only change the reputation of a single post once, ever. You can only give rep points a certain number of times a day.
Keeping those two facts in mind, when you go through a thread and see a post with, say, - 7 rep points, it means seven seperate people thought it warranted a negative rep.
It could be more - perhaps 10 people gave negative rep, and then three other people who browsed the thread later didn't think it deserved it, so added a positive rep point each, leaving - 7 rep. I'll often be reading a thr