I think a lot of it is luck. We've all seen some topics that are lightning rods for replies (religion, Napster, etc.), but assuming you're not counting the chat threads, the others seem to be kind of random. I can't cite a specific example, but a few times I've noticed threads that didn't go anywhere, and a week or so later the same topic in a new thread gets tons of replies.
For a great recent example of a thread that caught on for no obvious reason, look in the Distributed Computing forum for "ignore this thread".