There are some dogs at dog parks who give of energy that causes other dogs to pick on them--esp. around 1 year old. My male, when he was young and insecure, was like that. Once he was lying on the ground in the shade by himself catching a breather from play when a random dog pounced on him with ill intentions. I immediately pulled that dog up by the scruff and sent it on its way with its mortified owner. I carefully policed interactions in the dog park. If bad dogs arrive--and you can feel the energy of the park change--we leave. If there are dogs who are minor annoyances, we stay on the opposite end of the park and avoid them -- going with friends with stable dogs makes this easy.
My dog's energy changed as matured. He grew up to be gregarious, sociable and enjoys dog parks and play care -- he wasn't damaged. He grew out of being targeted as he got more confident and bigger. He's not FA or DA. However, we monitored interactions at parks closely, shooed away or avoided nasty dogs, planned play sessions with friends at the park, and paid for a lot of supervised play care with carefully chosen, temperament tested dogs that he got to know well. We also built his confidence up through obedience training.
He's a great pacifist -- never been in a true fight in his life, and he'll try hard to avoid from dogs that are aggressive--he's naturally pretty submissive. However, when an off-leash ankle biter ran from it's owner and lunged at my leg on when were out walking on a sidewalk, he surprised me by jumping between me and that dog, pinning it with a paw, and putting his jaws over its head to hold it--without biting down. It happened in the blink of an eye before I even had time to react. But then he paused, and he looked at me and asked with his eyes, "Um, what do I do now?" He released the terrified nasty dog with a "leave it" command--because he trust me.
I'm going to repeat what I've said several times one last time: protect your dog at the dog park, not just physically, but also in terms of the company it keeps. Mine grew up fine with other dogs because I did that. I tried to be very thoughtful about the kinds of dogs I wanted mine interacting with, and created opportunities for that to happen. Stable, sociable dogs teach younger dogs to be stable and sociable. Unstable, nasty dogs teach younger dogs to be unstable and nasty. It's that simple.