Why has my dog started getting into fights at the dog park?

Why has my dog, who used to love other dogs, started to get into fights at the dog park?  Is he dominant?

Dominant?  Usually not.  Few dogs are born wanting to get into fights.  Fighting behavior is evolutionary suicide.  So why has my dog started barking, lunging, and biting other dogs at the park?

There could be many reasons -

1. He's not feeling well.  Dogs are very stoic and hide discomfort very well.  Make sure he's been recently vetted and isn't suffering from pain or illness.  If he is ill, dealing with the medical condition can often make the behavioral issue go away.
2. He's been punished by or around dogs - One of the dangers of using punishment training (leash corrections) is that the punishment is often paired near or around other dogs (leash corrections for looking at dogs, "training" classes where leash corrections are done around other dogs in class). This can also happen at dog daycares/dog walking services that use punishment (spray bottles, physical corrections, bark collars, etc.). Pulling while on leash can also cause this (the dog sees a dog on the street, gets excited, pulls towards it, and experiences neck pain and frustration - in a dog's mind, the other dog is causing the pain).
3. He's been harassed by other dogs at the park - Look closely at the picture of the Beagle.  Does he look happy?  He's doing everything he can to get away from the pushy Ridgeback.  If the Ridgeback doesn't stop, the owner of the Ridgeback doesn't recall his dog, or the Beagle owner doesn't leave, how long would it take before the Beagle decides to bark and lunge to send the Ridgeback away?  How long would it take before the Beagle decides he hates the dog park, hates Ridgebacks, and hates all dogs? If the Beagle fights back, the Beagle owner collects his dog and leaves - reinforcing the very behavior of fighting back.