Here is this thread from 2014.. saying exactly the same thing:
https://www.coh2.org/topic/28161/new-pc-restarting-bug-yes-you-read-that-right
Ok, so as the title says, was doing some 3v3's with some friends and all of a sudden, my PC restarts out of the blue. No error message, no hard-locking, nothing. It was as if the restart button was clicked (which it certainly wasn't).
Yep... exactly this is happening to us.. so going back a few months in the bug forums I dont see it happening for anybody else in the recent history. (Side note: Since no bugsplat screen is possible there is no way to report it either.)
So, the question is; is this a thing.. or is it time to go down the 'lets update hardware' rabbit hole?