netmeg - 2:10 pm on Dec 24, 2012 (gmt 0)
Traffic quality possibly - do you know where your traffic is coming from? Layout issues - if they think there's a chance that you're getting accidental clicks, they will roll them back. Also, advertiser issues. I had some taken away and I questioned it and was told the advertiser either broke the rules or didn't end up paying for his clicks.
Not much you can do about it, except make sure your traffic is coming from the expected sources on the expected keywords etc.