LOL I think you are very lucky your bird seems to enjoy so many people! Mine will bite the fingers of any strange fingers that go into his cage (he's a grey).This behavior is actually quite common. They tend to be territorial of their "flock."
It could of been anything from the color of this guys hair or clothing, to the guys demeanor/stance. Maybe in his little mind, this guy LOOKED like the enemy. I wish there was a way to figure out what goes on in their little bird brains LOL
P.S. my neighbor always insists on putting his fingers into my birds cage, even after several nips.