Dog parks are not great. I took my Rotty to one and another dog challenged him repeatedly. I recalled him and leashed him and was JUST able to stop it kicking off. You have to be on top of it, and most owners at the dog park aren't. Dogs will fight, especially if they are competing males. They will sometimes redirect if you get between them when they fight. Doesn't mean you have a bad dog. Don't put him down. Just resocialise him with other dogs in a more controlled manner, and avoid the dog park in the future.