There appears to have been a Penguin data update either yesterday or early today.
What makes me so sure?
Well, I set up a little Penguin trap. I built a one-page site, tuned for a nonsense term, and built about a dozen links to that page with the nonsense term as anchor text.
Then, I waited until it ranked #1 for that term…that took just a couple of days. The pages that linked TO that page all rank for that term, as well.
So, once my nonsense site ranked #1 for that term, I disavowed all links.
And waited. Till today.
BOOM. After not moving a single spot for the past month, it suddenly dropped 9 places.