It seems that Niantic have started using Google’s Recaptcha tool to stop robots in Pokémon Go. The tool is built into Pokemon Go and will site a picture of Magnitude with a tick box that says I am not a robot below it.
There are a few things you can do to trigger Pokemon Go to suspect that you are a robot. Don’t of these things include traveling long distances regularly or performing lots of sequential evolutions on a short space of time.
My suspicion is that since the Recaptcha tool is made by Google, it would make sense if other internet activity that Google associates with your Google account could trigger them to think you are a robot. For example, comment spam while using a computer you’ve logged into Gmail with.