Did they tell you the reason other than just invalid activity? Any idea what caused this sudden surge in traffic from Google, Bing and Yahoo
No, risk of generating Invalid activity for advertisers was the only reason.
Traffic from Google: I got #1 ranking for a keyword and traffic spike. I am currently in #3 and getting similar traffic levels. Obviously when a high paying keyword ranks #3, won't your CTR and page views go higher?
Traffic from Bing: Temporary surge in 1 day. I inspected all IP referrers, city/state addresses and send them to Google. I didn't detect any 1 or 2 IPs trying to do funny stuff on my site. Even if there is someone who is trying to click-bomb my site, is this my fault? What should I do to protect myself?