1) Genetics
2) Hormonal Imbalance (Thyroid gland is overactive or under active)
3) Stress
4) Chemicals in Shampoos, Conditioners
5) Part of an underlying disease such as Lupus and Diabetes
6) Have gone major surgery
7) Taking Medicines such as blood thinners (also called anticoagulants), medicines used for gout, high blood pressure or heart problems, vitamin A (if too much is taken), birth control pills and antidepressants.

Hairstyles that puts too much tension on your hair like Pigtails, ponytails (secured by elastic bands), cornrows or winding too tightly onto rollers (especially heated rollers)
9) Chemicals used in permanents (also called "perms") may cause inflammation (swelling) of the hair follicle, which can result in scarring and hair loss.
10) Scalp conditions such as Fungal infections or Psoriasis.