Cats will pull out clumps of their own fur when they are stressed or anxious as a compulsive behavior. Cats do groom themselves, but not to the extent that their fur falls out in clumps.
A compulsive behavior is when a being takes a normal activity and performs it to an extent or in a state that interferes with the being's overall functioning. This is what happens when cats compulsively pull their hair out. Most cats develop compulsive behaviors, such as pulling fur out, when they are young, typically fewer than 2 years old. Some breeds, such as the Siamese cat, are more likely to develop compulsive behaviors.
Before assuming that a cat is pulling out his or her hair compulsively, an owner should take the cat to the veterinarian's office so that tests can be run. Medical issues, such as scratchy skin because of fungal infections or allergies, could be the cause of the behavior.
If the medical conditions have been ruled out, then the owner should look at the environmental and social conditions in the home that may be fueling the behavior. A move to a new home, a new person in the home, a new pet in the home, separation anxiety, loud noises and attention seeking can all be root causes.