This is difficult to say, has your dog been raised with your cats from an early age? Does he have a long history of getting along with them? If so, I am thinking this was just a very freak accident.
Your cat may have fallen off the fence and your dog may have tried to catch it our of instinct.
Reminds me of my dog who has never attacked our chickens or baby chicks, but one day a bird had entered our home and in a panic flew right down the chandelier and my dog lunged up to catch it with his mouth instantenously killing the bird. The look on his face right after the fact was "oops.'
It could be your dog acted up on instinct, trying to catch the cat from the fall and not knowing it was your cat. It could be he also thought this was an animal that was invading his property and failed to recognize it was your cat.
A cat doesn't fall from a height like that often so in this context your dog may have acted too impulsively. It would have been interesting to know if your dog shook the cat with his head side-to-side which is more in compliance with instinctual prey drive.
Of course, this is just an assumption as we can never really know what was going on in a dog's mind. There is really no 100 percent way to know if your dog may ever harm your other cats.
So sorry for your loss.