system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do games go crazy when alt-tabbing?

When I minimize or switch to a different tab, it just freezes and turns to black before I see my desktop.  
When I switch tab back to my game, sometimes it goes back flawlessly (especially when windowed). Sometimes, it takes a full minute before I can see my game again.
gpt
You have your in-game settings configured for Full Screen, which means your video card is only rendering the game and is no longer rendering anything on your desktop for that screen.

When you alt-tab in full screen mode, the video card has to drop rendering for the game and switch to rendering the desktop again, which is why everything goes to black and depending on poor/lazy code in the game, can cause a crash or take an extended period of time.

This is why Borderless Windowed mode exists, the video card continues rendering the desktop behind the game, so alt-tabbing is very seamless while still providing a full-screen look.