Dog on Dog aggression by selection?

My adult 2 yr old female Boxer is super friendly, loves people and kids especially and gets along with most dogs including our other Boxer and toy breed dogs. But at the Dog park she

is fine with most dogs, but not all, she seems to pick another dog to dominate, and then pick a fight. I did not raise her

but I can say she will pull on the leash & lunge at other dogs

when walked, at the dog park of course she has free reign and

we watch closely, and out of 10 dogs she plays there is always

one that will push her buttons and trigger her aggression and

then thats it for the day most of the time we have to leave because she will play with others but go fight with that once

she already had a moment with them. She has not got into

a bad fight, mainly its been just her knocking the other dog

onto its back and her holding him/her down. But I dont want

any fights of course. And its not same sex aggression .

Today a puppy tried to mount her and after that we had to go.