If it's an off leash dog park your dogs should be off leash. Leashes impede body language and it's a recipe for misread signals to have yours leashed if the others are not. Misread and misgiven.
Myself, I am wary of dogs in a preformed pack. And sorry, but if one was a Rottie and I did not know dog or owner I'd be keeping a wide berth with my single dog. I know I'm judging by breed, but also by owner. I don't know if that new person is a knowledgeable owner or not, I act in favour of my own dog, not the other dog. You may have been encountering people of like mind.