These searches are populated from both your browser history (which we've already covered in this tutorial) and Google Suggest, a feature that "suggests" relevant search terms based on what you've already typed in.
The first thing I saw when I booted my PC yesterday evening was a notice that Google had prevented my default search setting from being changed.
If you've ever used Google to search for something, you know that the Google search field (whether it's the Google search box, the Google Toolbar, or your browser address bar) keeps a running tab of what you've searched for.
As you keep searching, Google suggests search terms based on the first few letters of your previously searched-for terms in order to save you a little time.
It took all of about two seconds to realize that Windows Search Service attempted to change my search default.
Well, I can't prove it based solely on the Event Viewer logs, but it's safe to say the search service is the prime suspect.
This is happening on both my devices (Moto G - 6.0 and Nexus 7 - 6.0.1).