Mimicking his keepers and audience, perhaps?
Perceived advantage, more attention?
Bears, dogs, even cats will do it for food.
I remember a book in which the pigs began walking upright...
Good one about the pigs. I think it's nice to see the Brits. still need dental care. Look at the newswoman's teeth. Bottom set is pretty scary. As for the gorilla, I have to say his posture is rather unusual. He may indeed have some sort of pelvic anomaly.