Female pattern baldness is a LOT like male pattern baldness.
Everyone has androgenic hormones in their body, like DHT (read all about it here), and some people, women included, are just genetically predisposed to be sensitive to these hormones.
More specifically, the cells in their hair follicles are sensitive to these hormones.
Over time, they get weaker from DHT. Instead of producing long, thick hairs, the DHT hormone shrinks the hair follicle, causing it to eventually die off.
In either men OR women, hair loss from androgenetic alopecia occurs because of a genetically determined shortening of the hair’s growing phase. This is called the “anagen” phase. The time between the shedding of the hair and the start of a new anagen phase also gets longer. It takes longer for a new hair to start growing back after the previous hair has been shed.