Autumn and I go to the local dog park a few times a week. Most of the dogs are "regulars" that come daily. I've never heard of nor seen any dog becoming aggressive from a dog park. Are you sure it was aggression? A lot of people mistake simple dog talk for aggression. Sometimes and older or more solitary dog needs to be stern in order to keep a pursuing pup off of them. Also, sometimes dogs get a bruise or a sore ear and become agressive when other dogs sniff and bother the sore spot. I don't think dogs should ever be allowed to form and run around in a pack at the dog park. At our park everybody is within sight of their dog at all times. The dogs are not released at the front gate and left to explore all 17 acres on their own, they basically stay in the same area as the owners. I guess one of the main pluses at our dog park is strick enorcement of the rules and having somebody monitoring the park each day. Basically in our area those people who train their dgs properly and take care of their dogs use our park while those who don't take proper care of their pets don't use the park at all.