If you're connected to the internet via any form of public proxy server (such as a pub's free wi-fi, a VPN or an anonymous proxy in another country) Google sees all traffic via that server as coming from the same place and queries it as being due to possible automated searches.
So, for example, if people in Wetherspoon's pubs all over the country start googling at the same time, Google sees all of those queries as coming from The Cloud's server (because that's who provides Wetherspoon's free wi-fi) and treats them all as if they'd come from a single web user.
You might not think that you're using a proxy but if, for example, you use security software that checks out the safety of each link Google offers you (such as that which comes with the free version of Avast) all of your searches are routed via the servers of that security software, rather than being sent directly.