Because the big dog didn't feel like being sniffed by a young'un. Dogs are not always nice to each other, and puppies sometimes get put in their place.
Protect your dog. Keep interactions positive, only interact with dogs you know and you are sure won't do this.
Relax and don't be anxious yourself. Your nervousness travels right down the leash. Go to places where *you* feel happy and confident, let him watch, but it is not necessary to have him interact. If he races ahead, do an about turn, with the happy voice, and lure him with a treat. Bring toys on walks. Keep them short and fun and always end on a good note. If he even starts to think of barking, distract him.
And forget the alpha thing and become your dog's protector and benevolent leader.