A dog has body language when it wags its tail it is saying be friends with me or "I'm happy" but when a cat wags its tail it saying "stay away" I'm ready to fight, so the cats best instinct is to fight first and the dog fights back, and the cat, and the cat, then the dog, and well you get what you call a "cat-dog-fight"
There are different species and they just don't get along with different species.