Maybe she somehow thinks she is protecting you or not wanting other dogs to be near you, you said she didnt act mean to the puppy until you came up to greet the new dog owner?

My Rott is friendly with all dogs but when our neigbors (Boxer) dog is on the other side of the fence and I go to pet him he gets ferocious with the Boxer, but... then the dog comes over to play and he is frollicking with him, the only time he acts that way is when the dog is on the other side of the fence ... go figure.
Sorry i wasnt of much help

Oh and he does the same thing with the Danes behind our yard, only when i come to the fence he gets nasty to the other dogs, when i go away he is friendly again. And he only does this when they are behind fences, so maybe that is somewhat similar to the other dogs in your situation being on leash.