Google peers with many national and regional ISPs at exchange points. I don't know the exact numbers, but I'm guessing a fairly significant portion of their traffic does not go over traditional "tier 1" networks and rather is handed off directly to ISPs.
Just a significant number deciding it is time to back up their content.
And at the backbone level it is all charged by the Mbps for example http://www.level3.com/en/legal/ip-traffic-exchange-policy/ http://blog.streamingmedia.com/2014/02/transit-works-costs-i... So a jump in a significant portion of the user base from a few MB per user per day to several GB will be several thousand times more expensive for that day.