aristotle - 11:10 pm on Jul 20, 2013 (gmt 0)
Well the Penguin demotion seems pretty clearcut. The rest of the lost traffic could be due to a combination of smaller effects, such as
(A) Your site could be among the many that have been gradually losing traffic as Google continues to push large well-known megasites and brands higher.
(B) When you make changes to a site, even if they are improvements, usually the initial effect is a temporary drop in rankings and traffic. If you keep making new changes, it could keep pushing you down again and again.
(C) If you got backlinks removed, or disavowed them, because of Penguin, the whole site is weakened due to the loss of incoming link juice.
So with so many potential factors involved, it could be hard to separate out their individual effects in a clear way.