A similar thing happened to me at a dog park once. We were visiting some friends in the lower 48 and found a small local park where people were very friendly. We'd been going there for a couple weeks and all the regulars knew us and knew Rigby. He does have a herding instinct but is extremely obedient.
One evening a man walked through the dog park with two dogs on-leash and one off-leash Aussie. Rigby approached them and the two on-leash dogs went nuts barking. I called him off. When he ran back to me the Aussie followed, they introduced themselves, danced a bit, play bowed, and took off in a game of chase. Meanwhile the guy with his other leashed dogs is walking towards the exit, screaming at his dog to come. Of course the Aussie wasn't listening, so the guy started shouting at ME, yelling at me to call my dog off.
I wasn't impressed, but I called Rigby anyway. Rigby stopped playing and started heading in my direction, but his travel path was unfortunately near the guy. Meanwhile the Aussie was still play bowing, trying to get him to chase again. As soon as he walked near the guy, the guy kicked him, hard, in the ribs. Hard enough for him to yelp and squirm away with his tail tucked.
I was livid, but I was more concerned for Rigby that anything. There was one other person in the park at the time, a local, who started tearing a strip off the guy for kicking Rigby. All the guy did was yell back at him that we needed to "keep our godda*$%& dogs away" and "if a dog is on leash it means to stay away" and "he has a right to protect his dogs."
Well, that guy was an idiot. If he knew his dogs were dog aggressive, why walk them through an off-leash dog park? And why keep the Aussie off leash? It still pisses me off thinking about it. Luckily we never interacted with him again, but it's those types of incidents that sours me to the idea of dog parks in general. Most of the time, the dogs are dog parks are perfectly friendly. It's the idiot humans you have to watch out for.