Our 5-year-old neutered male cat, Yazi, has suddenly become fairly aggressive towards our other two cats, a 3-year-old neutered male and a 3-year-old (estimate by vet) spayed female. Now, he's never really gotten along with Thunder, the girl, to begin with, but now any time he see's either of them puffs up and starts to growl and will hit them if they get close enough to him. Sometimes he even runs after them to hit them if they back away too quickly. I really don't have any idea what could have caused this, or what we can do to get him to stop. Any suggestions or advice would be greatly appreciated.