I sit at the back of the bus because I want to have eyes on my surroundings, don't like folks sitting behind me.
I sit at the back of the theatre: same reason
I sit facing the doors at restaurants: same reason.
I have no idea where this practice stemmed from but its something I do subconsciously at times.
Perhaps innate survival instincts ? Who knows?
I've always been like this since a kid, I hate and will not allow a mf to stand behind me.