Some cats end up never getting along. She could be asserting her dominance as cats are very territorial. Something else is going on and she has redirected aggression, or he isn't feeling well and she senses it. Also, any new cats sneaking around outside? There sould be and you don't know it. SOmeone new move in around you, a dog, a kid? WHat about a change at home? Move any furniture? Check out the behavioral tips at
www.ddfl.org. Also, look up Feliaway. It is a great natural product that works for many cats!