The most common causes of cats being aggressive towards people are:
- Arousal from unrelated stimulation. Most frequently, owners that are attacked by their cats are innocent victims. A cat is aroused by something and acts aggressively towards interruptions. This is called a “redirected aggression” and is very similar to behavior in human relationships.
These attacks usually appear to have no reason because the cause is not related to the person being attacked. Several things can wind a cat up: seeing other cats or prey animals outside the window or conflict with other cats or humans in the household. Sometimes the attack may occur hours after the initiating event.
In certain cases, sources of arousal can be limited, but a big part of the solution is to notice why and when your cat is aroused and avoid bothering him at these moments. You can learn more about redirected aggression here. 
Fear. If your cat is afraid of you (or your veterinarian), you can expect him to be aggressive. It’s a simple defense. If a cat is not able to back away from a threat, it will attack. Only a few cat aggression cases are initiated by the cat. Usually, a cat attacks out of defense, especially if he is cornered by the person being attacked. If an owner tries to pick up a fearful cat, he is likely to be scratched if inattentive. Fear is also a common reason why cats are aggressive at vet clinics. Good news; if the cat is able to escape the fearful situation, it won’t attack you. Therefore, identifying fear sources and levels of fear are a key component of solving aggression caused by fear.- Pain and discomfort. Pain is another popular trigger of aggression, and not only in cats. Note that even mild pain, such as carelessly picking up a cat, can cause severe aggression. If you step on your cat’s tail, he may scream and run away. Or he may scream and attack you. If your cat is recovering from an injury, or is in pain due to another medical reason, careless handling is likely to cause aggression. If your cat reacts aggressively as you touch a specific body part, this may mean there is a painful spot, and your cat likely needs a vet’s assistance. Pain is usually accompanied by fear, as mentioned above. This type of aggression can also be learned, which is discussed next.
- Being aggressive is useful. If being aggressive helps your cat to achieve something, he soon learns that aggression is a great tool. For example, if your cat bites your hand and you instantly release him, he will understand that biting hands is the easiest way to get released. If you try to pull your cat out from under the bed but get your face scratched and go away, your cat learns that scratching is useful. In similar cases, a cat may become aggressive even before anything happens just to be sure it won’t. The basic solution is to solve the underlying cause of aggression and never let an aggressive act interrupt what you are doing. You can stop, but do not release your cat until he becomes calm. To achieve this, knowing how to hold an aggressive cat is very useful.
Inappropriately directed hunting or playing. If your cat stalks and attacks your ankles, toes or other body parts, it is likely he sees them as prey or a toy. This is one of few cases when cats are offensively aggressive towards people. Offensive aggression can be distinguished by different body language prior to and during the attack. A fixated look, low profile walk, and forwards facing ears are the most prominent signs of offensive aggression. An important part of stopping such behavior is to redirect this energy towards toys, and avoid playing with your cat using fingers or other body parts.- Intolerance to petting. This is a specific case of cat aggression towards people. A cat attacks a hand that is petting him, seemingly out of nowhere, even if the cat enjoyed the petting few seconds ago. A plausible explanation for this is that your cat indeed likes being petted, but when it goes on for too long, becomes too intense, or gets targeted towards an undesired body part, he shows objections. Usually this is preceded by some subtle body language signs, such as tail or skin twitching. The key is to stop petting before those signs appear. Over time, your cat’s tolerance level might increase.
- Territorial and sexual aggression. Cats rarely direct their sexually-related aggression towards people. Some owners report cats humping their leg or hand and then biting it. However, this is not very common and can be ceased by neutering. To some degree, cats can defend their territory from people, however, it’s hard to draw a line here. A cat is unlikely to think of you as a rival who will steal his food or women. More likely he is doing it because of stress and fear, as mentioned before.
- Queen defends her kittens. This is known as maternal aggression, and obviously applies to you only if you have a female cat with kittens. Cats are known to try and avoid physical conflicts whenever possible by backing off and running away. A queen defending kittens, however, is not able to run away, and therefore may attack any intruder, even one several times larger than her.
Fear. If your cat is afraid of you (or your veterinarian), you can expect him to be aggressive. It’s a simple defense. If a cat is not able to back away from a threat, it will attack. Only a few cat aggression cases are initiated by the cat. Usually, a cat attacks out of defense, especially if he is cornered by the person being attacked. If an owner tries to pick up a fearful cat, he is likely to be scratched if inattentive. Fear is also a common reason 
Inappropriately directed hunting or playing. If your cat stalks and attacks your ankles, toes or other body parts, it is likely he sees them as prey or a toy. This is one of few cases when cats are offensively aggressive towards people. Offensive aggression can be distinguished by different body language prior to and during the attack. A fixated look, low profile walk, and forwards facing ears are the most prominent signs of offensive aggression. An important part of stopping such behavior is to redirect this energy towards toys, and avoid playing with your cat using fingers or other body parts.Finding a cause is the first step in solving aggression in cats. In some cases, it may be the only thing; for example, if a cat’s aggression is caused by pain, treating it medically may be enough.
More commonly though, more than one cause is involved. For example, stress is commonly associated with aggression but not on it’s own; it usually involves fear, discomfort, and arousal. Besides, a cat under stress can learn that aggression helps him to be left alone.
Next in our cat aggression series, you will learn to recognize the body signals and warning signs of aggression in cats.
