Reproduction:

1. play the game in fullscreen
2. make a screenshot by pressing Win + Print
3. 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.