Though I haven't experienced it yet with a cat...knock on wood...I've heard about a lot of cats getting over-stimulated when being pet, as Jamie talked about, especially along their back and by their tail. My female cat is very docile with family, but if a vet touches her hindquarters, she hisses And growls. I hear that is common and it is an area they are more sensitive about. Not sure if that relates to mating at all, or what that is about. However, if the aggression is very random and unpredictable, and not just tied to petting, you can be looking at many things, from non-convulsive seizures that alter behavior to thyroid issues. There are usually subtle clues for these sorts of medical problems, so if he is showing really, really random aggression, the more details you can give the better. If it's all about the petting though, try to find cues that indicate he is about to show aggression (for some cats, it is tail lashing, some a change in eye dilation
, etc). )amie's tip is excellent, that is, using treats and other things that comfort your cat during petting sessions, and slowly lengthening sessions while rewarding calm behavior. Sounds pretty nerve wracking to randomly get chomped on!