I don’t think there’s any way of proving your theory. Many times I’ve seen the dog park used as an excuse for poor genetics. People always look for a “reason” that the dog is the way it is and always try to put the blame on someone else. Many times, they think they did all they could in finding a good breeder, so it can’t be genetics…it has to be an experience in the dog’s life at some point. Most people still really believe that it’s all about how you raise a dog that makes it the way it is…and they don’t believe that genetics plays a role in that. I’ve personally seen how much greater the genetics are as a piece of that puzzle than the raising, and how a dog will revert back to its genetics when its finally allowed to, so I don’t ever believe that a terrible incident at a dog park will cause unreversable fear/dog aggression.
My dog was attacked as a puppy by a neighbor’s dog. He got bit on the face pretty bad, no issues with other dogs to this day. He wanted to play with that other dog the very next day. He would get rolled/dominated at dog parks when he was younger. Has no issues going to a dog park today, except that now he is the dominant one that would do all the rolling…so we don’t go because I’ve learned how big of an affect that can have on a weaker nerved dog.
I think what you see is that in shelters, many times you have mixed breeds. Mixed breeds who’s genetics don’t line up. I know the most popular mix around her is a lab/pit. So you’re talking the calmer/weaker nerve of the lab, mixed with the aggression of a terrier. Just not a good mix.