I would say it was a warning that he will attack you from behind. We have a roo who is a little neurotic and freaks out when someone wears different clothes/shoes in the coop.
Some people say holding them upside down teaches them who is boss, others say to knock them down if they jump at you and then hold their face in the dirt. Lots of different ideas. Ideally you don't want him to be your best friend, but you don't want him to attack you either. A healthy respect of each other goes along way.
