It's called HDR, High Dynamic Range. You can thank Valve for implementing that into videogames.
This. I like it when it's subtle, but games can't perfectly replicate the real effect yet so it just keeps happening over and over again. In reality, it only happens in real life when you move from a dark indoor environment to a bright or outside environment. Once you are outside your eyes adjust and it stays the same. But yeah, it can get pretty nauseating how often it swings back and forth in video games.