My dog had this happen to him last week. We went out in the forest for a hike. We bumped into an owner with his 1 year old Golden Retriever. To date, this gentleman has not done any training with him and told me that he has started looking into classes because his dog will not listen to him at all.
Now this dog is EXTREMELY friendly, happy, lively, and completely ballistic with excitment.....ALL THE TIME!
He and Sonny were running around playing (they've played before), when out of nowhere he mounts my dog and starts going to town. Did I mention he was going to town?!
Sonny turns around and looks at him like....What The H..?! The guy is yelling at him to knock it off (of course dog is not listening), and then Son takes him down. No growling, showing teeth or biting, he just whirled around and body slammed him to the ground. This dog was totally excited by my dog. They contined to run and play but the dog kept sticking his nose in my dogs butt and would lick and mount. Sonny kept knocking him to the ground.
We left a little while after that.