Twitter also flags accounts that only ever unfollow those that don't follow back. This is extra annoying as it doesn't happen immediately & you will have already invested a lot of time building your following, plus Twitter don't seem to unsuspend accounts that are caught out in this manner. To avoid this you need to also unfollow some of the users that did follow you back. This undoubtedly leads to some people unfollowing you as a result, so try & target the users you unfollow - I usually unfollow those that haven't tweeted in the last three days. This does only appear to be an issue with new accounts though, once your account is three months old you no longer need to do this additional unfollowing.