That is a good question. The answer isn't really one or the other. It's probably both. Studies show a higher rate of vertebral fractures in the year following a vertebroplasty compared to patients with the same problem who don't have the vertebroplasty.
It looks like increasing age, rate of falling, and other health issues add to the risk of re-fracture. Early studies reported as high as 52 percent chance of refracture after a vertebroplasty. Those numbers seem to be declining quickly as doctors gain experience with this treatment option.