Almost the exact same thing happened to us around May of last year. What had been a hugely profitable campaign for more than 8 years tanked in a matter of days. We still haven't figured out all the reasons or totally recovered, but there were a few big ones we noticed, at least for our market/campaigns.
- Content clicks on mobile devices became a huge portion of our clicks, and they're pretty much worthless (99%+ errant clicks that would immediately bounce, if they showed up in our analytics at all). We disable all mobile content clicks now until they add swipe to click or something to make them even a little useful.
- "Synonym" matching seemed to grow way out of control to things not remotely related. We mostly fixed with the broad match modifiers (+ before keyword)
Those are the only two we've really nailed down, and they helped make it at least profitable again, but still not anywhere near what is was before the week it tanked.
- Content clicks on mobile devices became a huge portion of our clicks, and they're pretty much worthless (99%+ errant clicks that would immediately bounce, if they showed up in our analytics at all). We disable all mobile content clicks now until they add swipe to click or something to make them even a little useful.
- "Synonym" matching seemed to grow way out of control to things not remotely related. We mostly fixed with the broad match modifiers (+ before keyword)
Those are the only two we've really nailed down, and they helped make it at least profitable again, but still not anywhere near what is was before the week it tanked.