Most excessive hair growth is due to a hormonal imbalance caused by conditions such as poly cystic ovaries. Other rarer causes are Cushing’s syndrome and congenital renal hyperplasia. Sometimes tumours may cause it or taking anabolic steroids.
Yes hormone imbalance is my answer too. Anabolic steroids are taken (illegally!) by humans wishing to increase their muscle mass, and act like male hormones. They can have devastating effects on the heart muscle, and also lead to excessive hair growth, even for females.
Seems science hasn’t been able yet to invent a pill that promotes head hair growth. Imagine how rich you’d become if you came up with a solution to baldness!