In general, this condition is due to a hormonal imbalance where there are higher-than-average level of male hormones in the woman's body. In particular, a common condition which can cause this imbalance is called "polycystic ovarian syndrome (PCOS)."
PCOS is a syndrome affecting a woman's ovaries and her ovulation with its main features being cysts in the woman's ovaries, high levels of male hormones produced, and irregular or skipped periods. Baldness, and increased hair growth on the affected woman's face and body can also be observed.