When cats are upset or playful, they tend to exhibit that kitten behavior that can be annoying at best and injurious at worse; biting and scratching. It is a natural instinct to pounce and attack. A cat may act or appear aggressive for various reasons.
Cats by nature are predators so their playful aggression is sharpening those hunting skills. The predatory instinct makes them hunt and kill. Even though domestic cats don't have to hunt since we feed them, they still need to practice hunting skills to fulfill their instinctual kitten behavior. You will often witness the long periods of waiting and stalking before the explosion of the attack. You may have been the prey object yourself when your cat has crept up to you while you were sleeping and attacked your foot or head. We often play with them letting the cat sit staring at our wiggling toe or finger, then pounce and attack. I had a Siamese kitten who loved lie in wait under my bed until my foot came close enough for her to reach out and grab it. Although she was playing to hone those innate hunting skills, her playful bites and scratches still hurt.
Fear and anger can also cause aggressive kitten behavior. Cats that have not benefited from socialization as kittens usually are more suspicious and fearful of humans. They enjoy being touched or picked up. They will show their fear of being handled by biting and scratching. Even socialized cats can become angry or frightened in situations they are unaccustomed to. Maybe they are afraid of a stranger picking them up. Some cats hate bath time and will literally fight tooth and nail to avoid getting in the water. Exposing them safely to new situations during socialization can help prevent aggression due to fear or anger. It is important to do this early on and on a regular basis since once a cat has been successful at warding off a threat with aggression, it will be more likely to act aggressively again.
A cat may show aggression to protect his/her territory. Again, this type of aggression is usually the result of poor socialization. Cats will fight furiously for their territory if it is encroached upon by someone or something that is not considered a normal visitor. When we brought home a new kitten, instead of introducing her to the two dogs right away, she had full access to the house on her own for a few days. Once the dogs were allowed back in, she just knew she had to fight for her life. Once she realized they were not a threat, that aggression subsided. Luckily, my dogs have raised several kittens and knew to just ignore her. A mother cat will put on a ferocious attack to protect her kittens.
The most confusing type of aggression involves a cat that lies quietly purring on the owner's lap being petted, but suddenly explodes into a biting scratching fit. Although it seems incomprehensible, there's a fine line between what the cat considers enjoyable petting and what turns out to be over stimulating, unwanted handling. He may enjoy a certain amount of petting, but then he reaches a certain threshold and strongly rejects any more handling with the aggression.
Sometimes health issues may cause a cat to suddenly act aggressive. Any sudden changes in behavior that you can't explain with the previous scenarios should be reported your veterinarian. If the vet finds no health issues causing the aggression, then consider it a kitten behavior issue and work on more socialization skills.
Read more about looking after your kitten and Kitten Health at => http://www.takingcareofanewkitten.com