Well, you had three keys to get in - one into the building, one onto your floor, and then another to get into your room. My college was in a really windy town on top of a river bluff and if you and the guy across the hall had their doors open, and windows open, the draft would suck your door shut if you left it open.
The doors to our dormrooms were automatically locking unless you twist the handle and disengage it. Most people would just open their doors and leave the lock on. They would then walk down to the shower and while they were in the shower their doors would get sucked shut locking them out.
There's also the situation where roomate forgets that you are on the floor and locks the door behind them without checking to see if you have your keys on you.
Just stupid little things like that that would lock you out of your room