Check out if the drop happened on 19-21th October. If yes, then you were hit by the spam update.
However, you have rebounced, while many were killed by 90%+. Doesn't seem like a site-wide-ish penalty.
Check the pages that have lost traffic and try to figure out why. Maybe inbound links, maybe on-page. Many were hit for PAA spamming and thin how-to.