How does the game calculate injuries. Some of them are weird and random. For example, I played a game and my starting QB went down on a sack. I figured he had ACL or something. Checked the player card and the man is depressed! I mean, I know that depression is a medical condition but how do you get that mid game?

They're pretty random... you can modify the injuries.xml file if you want to see some happen more outside of games and others inside of games.