There could be a few problems that could be causing this minimization. Like drunkninja42 said, it could be programs running in the background or notification. However, if you don’t see anything out of the ordinary running in the background. You may need to reinstall the game. If it happens on all games, it could be a driver issue. Try uninstalling the video card driver, reboot, reinstall driver, then reboot. Is your computer completely up to date? you can also try defrag’n your computer. Also, clean out your computer of dust and access dirt build up. A clean computer equates to a happy computer.
My guess it is a software issue. So its either windows, the driver, or the game itself.