Have you ever seen or experienced a group of dogs running after another dog? Maybe even yours? This is a very dangerous type of interaction and should be stopped immediately. If your dog is chasing or being chased, call him or her to you immediately and leave the park.
More than one dog chasing after another can quickly become a mob-like attack. The risk to the dog being chased is very high; dogs have been killed after being targeted by a "mob". Predatory instincts can come out in dogs and arousal levels are very high setting a dangerous scenario.
From an evolutionary standpoint, mobbing behavior is an anti-predator behavior which occurs when individuals of a certain species mob, or gang up on, a perceived predator by cooperatively attacking or harassing it.
To witness an actual mobbing is an eye-opening reminder that no matter what our domesticated dogs look like, or how sweet they are, certain behaviors are primal and can come out when least expected. Even if it looks like all the dogs in the group are having fun and the 'chasee' doesn't look particularly scared or worried, always intervene and remove your dog. Better to err on the side of safety in this potentially dangerous situation.