Oh, some of it is good common sense.
Like facing the bed so you see the door when you sleep, etc. (Gives people the willies, otherwise).
Willies hell, it just guarantees a clear shot. Don't want to get the powder burns on the missus when a bad guy comes a calling.
I knew a guy that couldn't possibly stand to have his back to any kind of door or entryway. He said it had to do with being in wrestling through high school and college. Apparently he developed instinctive reactions to peoples movements in order to be a good wrestler.
He said that having his back to a door would cause his instincts to fire off and it was really distracting to him. He was young, just out of college at the time, so may be it wore off after a time.