Hair loss that only occurs once a year in rabbits is caused by natural shedding. If the hair starts to regrow quickly, the rabbit is likely changing her coat.
Other causes of hair loss in rabbits include parasitic infections, urinary tract disorders and other illnesses. Hormone imbalances and fighting with other rabbits or animals can also cause hair to come out. Hair loss resulting from injury, however, is often seen in just clumps. If the hair loss in a rabbit is accompanied by other symptoms such as vomiting, lethargy, lack of appetite or abnormal behavior, owners should consult a veterinarian to determine the exact cause.