I cannot be sure of the cause because there are so many factors that will affect the situation. The biggest one is that the aggressive cat can smell or feel that there is something wrong with the other cat. Injured or sick cats are forced out of the pack so they don't cause a safety threat. Take your cats to the vet to be checked out because illness could explain the aggression. Also, are they both fixed? When it is mating season, unfixed cats can get very aggressive, even to their owners. Check to see if anything has changed around the area where they fight the most. Also, try getting them new toys that they can both use such as condos or scratching pads. Try reintroducing them like you would with a new cat. This can sometimes clear the air of whatever problem they had. Lastly, how old are the cats? Elder cats often suffer arthritis in their joints just like we do. They can become aggressive if the other cat touches a sore spot. Just walking to the food bowl could be painful. Ask the vet to check for this as well.