Hair loss can be caused by a host of factors including:

Emotional trauma

Stress

Hormonal changes during pregnancy

Menopause

Viral or bacterial infections

Exposure to chemicals, seasonal changes and allergies

However the leading factor for hair loss in both men and women is:

Androgenetic alopecia, also known as hereditary balding

The primary biochemical mechanism leading to hair loss is:

The conversion of testosterone to the more potent androgen
dihydrotestosterone (DHT) and the binding of DHT to its receptor sites in the
hair follicle