Can be caused by numerous different mechanisms which are listed below:
Alopecia areata - autoimmune
Hypothyroidism
Androgens - high levels of sex steroids
Nutrient deficiences - iron and zinc
Heavy metals - high levels of copper, cobalt in the body
Blood thinners - like ASA aspirin, warfarin
Chemotherapy
Severe illnesses
Childbirth
'Corn-row braiding'
Vitman A - high levels
Trichotillomania - pulling out of hair follicles