Scientists aren't entirely sure. It may be that the injury itself leads to death. It's more likely a sign that the person is in poor health or has frail bones. These factors can lead to falling and bone fractures.
To put this in perspective, studies show that death is rare after orthopedic operations. About one percent of all patients die after bone or joint surgery. Three percent of patients with a hip fracture die after surgery to repair the hip.
The biggest risk factor is age over 70 years. The second biggest risk factor is congestive heart failure.