Since your dog has never shown any aggression to small dogs before, I would guess that the Frenchie's barking was the difference. Your grey probably thought, hmmm, my "brother" does not like this creature, I'd better check it out. I would not muzzle him, just be extra cautious around small fluffies from now on.
Edited to add, one of my greys grabbed a small dog that got into my backyard somehow, so I understand how scary it can be!