In advanced Osteoporosis, a person sometimes develops fractures of their vertebrae.
This causes Kyphosis or a Spinal hump.
Kyphosis and its resultant bent-over posture is often associated with older women
(although men get them also). The word dowager is a term meaning a "dignified elderly
woman". Since many older women developed this sort of hump on the back people began
to call the back disfigurement a dowager's hump - even if the person with the kyphosis
was not an elderly woman.