You can get your account suspended for a bunch of random reasons. I think Tribune recently had his taken away because his requester quals had "obscene" names. Low accuracy on HITs is a more common reason, but what qualifies as "low" isn't clear- some people got tagged as having poor accuracy, when their accounts were at 97 percent, which is widely considered "Pretty OK". You can get suspended if Amazon tries to verify some kind of account information and fails (can happen if you change addresses).
Basically Amazon has a really really really long list of things you're not supposed to do, but they aren't terribly interested in telling you what's on the list.