Game suddenly requires integrity verification?

I am unsure of how common and why this happens. But it happens to me and some of teammates I played today.

What happened to me:

  1. Game content is suddenly frozen while you can still quite the game and look at the stats. It was100% of time.
  2. Nothing is multiplayer was available to me

before I 2 files found corrupted during verification. Not sure how this happen. But after that I did not drop anymore. Although crashes still happen sometimes.