There are many reasons for a cat to become agressive.  The aggression may be aimed at you or a particular family member or visitor, cats or other pets.  Often aggression is misplaced anger or anxiety.  It can also be from abuse or torment/taunting, inappropiate play (someone roughhousing the cat), cat removed from his/her siblings to early for them to learn proper gentle play, and medical conditions, as well as response to fear or loud stimuli.

http://www.geocities.com/Petsburgh/4114/aggression.html

http://students.depaul.edu/~nrohani/behavior.htm

http://www.vin.com/PetCare/Articles/Cats/PCF00151.htm

http://www.geocities.com/Heartland/Pointe/9352/behavior.html