Reproduction:
play the game in fullscreen
make a screenshot by pressing Win + Print
go look at the screenshot
Assumption:
Since Win + Print creates a screenshot of the entire, well screen, and not just of the active window, the game registers the pressing of the Win key as a loss of focus and therefore pauses the game, just to instantly unpause again after the screenshot has been created. The user does not see this happening, as the entire process only takes a fraction of a second.
This is usual behaviour in most games. Most gamers disable their windows key due to this. There are even keyboards available that allow disabling the windows key.
This is not a bug with the game.