One of my sites got hurt beginning of August, dropping from about 1,200 visitors a day to 400 a day. Another got hurt end August, dropping from about 2,500 visitors to 800 a day.
Both sites are back to 'normal' as from 23 September. Did not change anything substantial on either site. So just waiting it out is a good option, unless one is aware of something on the site that's not well appreciated by Google.
It would be much better in my opinion if these changes in rankings were applied in a gradual fashion, and not in the sudden jumps we are experiencing at the moment.