An abnormally curved upper back, or dowager's hump, develops when the bones of the upper spine (vertebrae) become thin and brittle (known as osteoporosis) and collapse on each other. Having collapsed vertebrae in any part of the spine results in a loss of height.
When the upper spine curves, the lower back makes adjustments to keep the body in balance, which may cause muscle pain. This pain often stops in 1 or 2 years as the body adjusts to its new shape.
WebMD Medical Reference from Healthwise