Happening to me too. And some friends too. Many posts on it on internet, nobody know what it is.
What we know: it's new, doenst seems to be specific at Intel or AMD plateform, neither GPU (Happened on Radeon users and on 9XX and 10XX gtx). It's not related to anti-viruses either, tested with Malwarebyte;Kaspersky; Esset;..
But what we know is that it only appears on Windows 10 PC that aren't updated to the "creator update". I also saw that most of users experiencing this problem are gamers, might be due to the over-CPU usage, or maybe a Windows ad for the Creator Update that auto-leave when a process is running in fullscreen.
PLEASE, if you're experiencing the same problem, write your pc specs, if you're using steam, your windows version, what anti-viruses are you using.
EDIT: We found the problem, it seems to be a Microsoft Office Activator. (Found via checking logs.)