Why are my cats fighting?

I have two cats, one spayed and one neutered. They have lived together in our house for almost 5 years, and they have always been good companions. Recently the male has become very aggressive toward the female. (He is bigger than she is.) He chases her around the house and pounces on her until we yell at him to stop. But recently he has begun to attack her. When they are put in the basement at night (where they always have been), after a half hour or so, there is a screaming cat fight in the basement. Last night I had to separate them to stop the aggression. Today they are both very, very docile, but staying away from one another. What could be wrong with my Toby?