The most common reason for closing a question is that it has become a “spam magnet”. Some spammers will debark on Fluther’s shores and immediately start searching for old Qs about whatever crap they’re selling. If a Q is old and the only action it gets anymore is fly-by spam, it’ll get closed.
A Q won’t get closed just because it gets heated. If the actual topic of the discussion has been thoroughly hashed out and it devolves into a brawl, so that one post after another has to be removed and no new points pertaining to the Q are being made, then we may close it. The tipping point is usually that we’ve just gotten sick of cleaning up the pissing match. We’re a pretty patient bunch, but…