Lots of people don't know how to use AdWords properly. Lots of mistargeted traffic isn't paid. I periodically get bursts of Chinese traffic on my fireworks sites. For a while, Google had problems with the targeting and was showing ads for Display Booths and whatnot.
And yea, it's not the publisher's fault, but it's the publisher's problem, because publishers rank lower than users and advertisers in terms of priorities. That's what you get in exchange for Google giving you code you can set and forget. That's the product they offer.
(Personally, in your case, your traffic drop off was extreme enough that I'd be looking for some technical problem, if you're absolutely sure your traffic hasn't changed in ANY way. But again - a heat map solution will help ascertain this)