Many sites were hit on 04 September, including mine. Possible, that was the date for Hummingbird update. I have many 404 errors for deleted pages from database. I have experienced that 404 or 410 is the same for Google, at least regarding the time to remove them from index.
Google, on their webmaster forum, says that 404 errors does not affect rankings, unless those pages are linked within site, but who knows.
Also, is possible that has been some sort of update and some scores has changed.
Anyone else recovered from 04 September?