Did they accuse you of click fraud? That's what happened to me. Some research uncovered that all it really takes is one person who's angry at you to go on your site and click your ads repeatedly. If you search around, you can find stories of disgruntled readers, angry exes, you name it, and none of them have happy endings.
If you knew the risk in advance, you could set up measures to prevent it, but of course those measures would be in violation of the AdSense TOS.
If you knew the risk in advance, you could set up measures to prevent it, but of course those measures would be in violation of the AdSense TOS.