Copyright infractions, fraudulent traffic, taboo content,any serious violation of Google's TOS, etc. would be my guesses.

Hi heartfeltms and Welcome to WebmasterWorld [webmasterworld.com]

There's a discussion about a problem some of us are having in ads not loading that might help: [webmasterworld.com...]

And another one about how to get help from AdSense: [webmasterworld.com...]

Also, when your site is being banned from Adsense, you receive an email notification about the reason. You can also check at your Adsense account : Settings > Status