If you compare 3 sections in your timeline:
May 1 to May 9
May 9 to May 18 (another period of 9 days)
May 22 to May 30
Can you pull in some more detailed stats as to what got hit? Money terms? Long tail? Any particular group of keywords? Any particular section of the site got hit? etc...
Also, did you use any ranking trackers? Do you know which search terms dropped on 1st and 9th?