Dominance or bullying?

My 7mo male rottweiler normally plays well with other dogs. However, when a dog starts to bully another dog, he dominates the situation by picking on the dog that was previously being bullied. He's just started doing this, so I keep a sharp eye on him and reprimand him as soon as he gets out of hand. Is this a stage in his development and what can I do to deter this?