I think it MOST common to have owners who have no control over their dogs. Who do NOTHING when their dogs misbehave (" Awww... aren't they cute?") A dog that behaves that way simply doesn't know any better because, they haven't been TAUGHT any better!
The dog parks that I've been to have rule. Overly aggressive behavior is a no-no. And, so are owners who cannot control their dogs.
We visited a park in Southern Cal while we were there, for a month, on a job. When took both our Danes with us. One day at the park there was a small shepard mix that was all OVER Ike. Ike is a 160+ lb pussy-cat. He is always the first to back-off from agressive dogs, cats... squirrels! But, this dog wouldn't leave him alone. She was humping him, he'd walk off she'd run at him and practically tackle him, she was biting him. It was jusBARELY short of a true attack on this part. Ike, bless his gentle soul, tolerated her... walked off, ran away, tried to ignore her. I'm trying to determine who is the owner, with no luck. And, finally I see that Ike has about had it! This dog has been so persistantly after him and now other dogs are joining in. (Fiar to sya that those owners stepped in). Ike is starting to snarl at this bitch. And I'm thinking, " this is gonna be bad, he is going to rip a whole in her, trying to defend himself and it will be all his fault".
So, I run over and place myself between them. Stupid I know... but, I didn't know what the agressive dogs laws were in that area and I was NOT going to have Ike facing some sort of punishment/quarantine. I get ahold of Ike and lead him off. Telling the other do "NO!". Finally see this woman standing there watching us... LAUGHING AND GIGGLING. OK, I'm fuming now!
I tell her that she needs to control her dog that the park has rules against this type of behavior. She laughs more and says there is nothing she can do to stop her dog. I'm starting to boil at this point.
I tell her to put her dog on a leash or I will report them. Still laughing (and I'm now considering a little violence of my own) she tells me that she doesn't have a leash! AAAARRRGGGHHH!!!!!
We are drawing a crowd at this point. Other dogs owners are backing me up! People are berating her for her dogs behavior and she finally has to drag her dog out of there. I say drag... due to the fact that the bitch was still trying to attack Ike. She was snarling and barking at him and I was keeping her off with my boot. Not kicking her... but, blocking her.
Bottom line... read the rules of your dog park. Everyone is required to follow them. If they are not being followed you have the right to report those animals and owners. Also, don't be afraid to speak up! I think we tolerate such behavior not wanting to commit a faux-paw or offend anyone. Those type of owners NEED to be offended! And, you might be suprised that others will support you!