It's probably because someone has reported you to the HP team. It happened to me too. Basically you are porbably repeating the same mistakes across your whole account. It should tell you the reason at the top of the hub, but sometimes it;s not very clear what they mean exactly. It could be that you used the RSS capsule and the rules have changed about that. Also it could be that you were linking to other hubs that are no longer classes as related.
There are a whole heap of reasons and the only way to really get an answer is to email the HP team asking why the hubs have been unpublished. The email should be in the top part of your hub where it tells you what they feel you've done wrong.