This happened to us last year but did not have a name for it and had no clue what to look for, but to start at the beginning. It was duplicate content (bad coding) and took 3 months to recover.
Got hit again this week and the timing could not get worse. Have not lost traffic because of the long tail search but lost rank in the top keywords which we were on the top. It could be over optimized, dup content or anchor text? Who knows, but more experienced at this than last year.