Game minimizes then full screens again a few mins into play

Hello, my game is always set to full screen. A few minutes into the game, the game shrinks into a minimized state for like half a second then re full screens again. What is causing this to happen with my video driver? I have not tried a full screen game in years due to Blizzard removing all full screen abilities of their games. I am happy to be back to a full screen however!