I have been noticing this happening a lot these days. Google Chrome has a feature named:
Predict network actions to improve page load performance
This is accessible from the settings (chrome://settings/advanced). I am not sure if this is enabled by default but I have it on.
And it does work nicely. I have seen web pages opening up in instant. The browser tries to guess what page you might open next and loads it in the background.
Here is how it tells you that the current webpage was actually pre-rendered: