When a new pet is introduced into a household there is a natural recipe for fighting. Sometimes this can take the form of a cat becoming aggressive towards a dog!
2. Cats like to establish and dominate their geographical territory. Once they have decided the borders to their territory cats will naturally want to dominate all other animals found within those borders - including dogs if they happen to share the family home. When a new cat is introduced into a home that already has a cat they will often fight a lot - and actually may both believe that they dominate the territory - resulting in a permanent tolerance but dislike for each other with sporadic bouts of fighting.