Causes of hair falling out in clumps include alopecia areata and telogen effluvium, as reported by Physician's Desk Reference. Both of these are conditions that can be medically treated.
Alopecia areata causes hair to fall out in small round or oval patches. These bald patches appear suddenly and may or may not affect the entire scalp. It can be a recurring condition or may only happen once, according to the American Academy of Dermatology.
Telogen effluvium may be the result of major surgery, prolonged stress, childbirth, illness or other reasons. This type of hair loss may not occur for several months after the cause has been eliminated, and in most cases, the hair eventually grows back.