Probably scraping with a sea of proxy IPs or a rented botnet
One could test this hypothesis by doing the same thing to it--spamming search queries from numerous IPs--to see if Runnaroo's IPs stop working (get 429'd or captchawalled). I don't really have time to whiteknight for G, but OP should be prepared for them to take notice. (Maybe create a global rate limit.)
I'm interested in how the ranking algorithm works because it seems really good.
Edit: Runaroo is now down. Looks like he was ahead of me on the global rate limit suggestion.