Google AdWords representative, Laura, said in a Google AdWords Help thread that the main reason there are "invalid clicks" on ads is due to publishers.
Yes - Google said that "usually" invalid clicks come from AdSense publishers. Either the publishers are:
(1) Clicking on their own ads or;
(2) Telling their friends and users to click on their ads.
Google's Laura said:
I don't think there's any connection between AdWords suspensions and AdSense ones. AdSense "invalid clicks" suspensions are usually just that: publishers clicking on their own ads or encouraging their visitors to do so. If you think you didn't violate that policy, contact AdWords support.
So there you have it - usually those invalid clicks are AdSense publishers fault.
Forum discussion at Google AdWords Help.
Image credit to ShutterStock for the fuzzy click.