Dominance, pure dominance . . . with maybe a little disdain on the side.
You probably need to watch him when he's around this dog in the future. Your GSD may have a reason to dislike this dog, or he may just not like him, but there is an outside chance there could be some aggression at some point. I wouldn't be too concerned, just be aware and don't let the two dogs be in a situation where something could happen.
My Bimmer treats the Lab across the street in somewhat the same fashion. The Lab used to come up here and harass Bimmer when he was a pup, but Bimmer grew up, and when he did, he put the Lab and the other four dogs in his pack in their place. I don't know what he did to them, but everytime they see him now they all tuck their tails and run. Now, if Bimmer sees the Lab sitting on the front porch, he walks over to the porch and urinates on each corner of the porch. The poor Lab just cringes against the door and shakes. He really shouldn't have tormented Bimmer when he was a puppy! I'd feel worse about it except that he also comes over and chases our cows. Bimmer scaring him into staying across the street may save his life, since dogs that chase cows don't live long in the country. It's really a shame, since when they first got him he was a nice dog, but now he growls at everyone, chases cars and chases cows.
So don't feel alone. Just be aware and be careful.