Most likely because a user uploaded something that Google didn't like. Google Adsense is pretty strict on the pages that you can display their ads on. If some

Most likely because a user uploaded something that Google didn't like. Google Adsense is pretty strict on the pages that you can display their ads on. If someone uploads a pornographic or offensive image to the site and theres a Google adsense ad on it, then Google will ban the entire site.