It's an over reaction on your part
Dogs live in a sensory world we can't imagine. While you were looking to find a place on the bus Rex and the other puppy may have been exchanging signals that Rex didn't quite take as friendly. Just a wild guess but the other dog was probably male?
Since you own the larger dog another problem us humans have managed to introduce into the canine world is... pure confusion! In Wolf society if you are small you are a puppy, and you are expected to be submissive. If you are large then you are obviously an adult. But look at what we have done to that strict social and simplified order with our dogs! I've seen 50 lb puppies trying to be submissive to 15 lb adult dogs. It's a sight to see. In your Rex's case he is a large dog at 3 yrs, possibly being challenged by signals you did not catch by this small dog. To him it's a puppy. The small dog on the other hand is a big adult wolf in a tiny dog's body. The other dog tried to stand its ground and things didn't turn out too well for it. You may have noticed it's the smaller dog that tends to be barking or making aggressive motions towards the larger dog than the other way around.
I think you should chalk it up as a chance occurance, but keep an eye on future scenarios where Rex meets up with a dog previously unknown. Likewise since he knows the other dogs in his class, it shouldn't be a problem there.