It's not uncommon for pain in other places to go away after a hip fusion. It's more common for joint pain caused by the unstable fracture site to be relieved by the new hip position. Relief of chronic back pain isn't as likely.
Doctors think the reason for the pain relief is caused by changes in position of the hip and spine. Improved stability in the area may take the load off joints and soft tissue structures above and below the fused site.