My donkey bites and chases my stallion!!!!?

Ok my donkey is a big baby. He has never shown any aggression to anyone or any horse we've had before, not even the little pony we had. We have this very sweet, gentle paint stallion. We got him about 6 months ago, for the past 2 months or so my donkey has been bitting, chasing and jumping on our paint. Poncho (our paint) doen't do anything to instagate the donkeys behavior. The donkey never leaves any marks on him. Is he doing this to show leader ship or is he just playing? If it's not just play what can I do to stop this?