Originally Posted by GoNe

There are no more Google dances now after they launched caffeine. There are so many factors to consider when trying to analyze ones drop in the serps.

If you know you are doing something that is considered blackhat or borderline (greyhat) that could be it. For example keyword rich nonsense text somewhere on the page which is the same color as the bg.

I know you said you didn't buy backlinks but another thing you could get penalized for is for buying backlinks from well known backlink sellers.