Very easy to happen. It only takes on dominant dog to start the fight and the others can snap into pack mentality...by holding and grabbing the legs /stomach.
I would NEVER take my dog to a dog park...ever, ever, ever.
Too many unrestrained dogs...too many training methods. Invigorated under exercised dogs are asking for something like this to happen. One dog overawed will become nervous , and switch on the 'weaker dog' scent. All dogs will be tuned into this. All it takes is that one scared dog to snap at a dominant dog and it's on.
Many owners are there for the chit chat and not watching the physical signs of growing tension.
I really think this is a negative that Milan has pushed...throwing all dogs in together. You only have to see how many dog fights occur on his show.