Apart from June 7th being a Saturday and at least on my sites (YMMV) it's almost a free for all on weekends lol :o, I think a change in SERPs can definitely impact eCPM. The reasons why people visit are in the details of the keywords they're using to find your site.
Did you lose positions on popular keywords? Or was there a shift in the phrases they are using to find the site? There's a good thread in supporters by incredibill about how there's seemingly a shift in the amount of keywords used to query search engines with, notably a decline in one and two word phrases.