*Heaney wrote, "Although bone mass is certainly the most extensively studied of the fragility factors, low bone mass is not the whole of the osteoporosis story and may not even be its most important component (despite frequent assertions to the contrary). If one could magically normalize bone mass in everyone, would one eliminate osteoporotic fractures? The best answer that can be given today is 'no.' There would be fewer such fractures, but there would still be many, especially hip fractures." (Heaney RP Bone Mass, Bone Loss, and Osteoporosis Prophylaxis. Annals Internal Medicine 15 February 1998 128: 313-314)_
As Heaney implied, What else besides intrinsic bone problems could cause hip fracture?
As the figure below shows (with a lot of artistic license), osteoporosis makes the bone intrinsically weak, simply by offering less structural mass
But there is more Old people get the dwindles. They fall more and when they fall, the risk of fracture is higher because they can't catch themselves. A risk of falls (from caused such as bad vision, say, or a neurological disease) is an independent risk factor for a hip fracture.
The key point to know and recall is that the amount of energy needed to break a hip is only a fraction of the energy available from a typical fall. That most falls do not cause fracture is a testament to the normally present energy-absorbing processes (catching yourself, basically).
So if you fall frequently and if you can't catch yourself as you fall, you are going to break bones even if those bones are intrinsically healthy.
A related point: that's why patients falling off the OR table is such a potential disaster: sleeping patients can't catch themselves! (and anecdotally, some of the worst fractures I have seen are in the inebriated.)