There are, of course, instances of hair loss that aren't caused by any underlying disorder. Hair loss can be a result of medical treatment – most notably chemotherapy.
Chemotherapy is a complex treatment where harmful chemicals are used to target cancerous cells in the body. The treatment cannot be specifically targeted however, and hair cells are also affected, causing the hair to weaken and fall out. This hair loss is total, but not permanent, so often people opt for full pre-made systems when suffering from this kind of hair loss.