Non-specific (85% and mostly self-limiting)
Infection (discitis; osteomyelitis; TB)
Inflammatory (Ankylosing Spondilitis, Psoriatic arthritis)
Congenital (Spina Bifida, scoliosis)
Degenerative (Degenerative Disc disease - Spondylolisthesis)
Neoplasia
Trauma (fracture)
Bone disorders (Paget's; osteoporosis; spinal stenosis)
Non-spinal/MSK e.g. Renal tract; Cholecystitis; Pancreatitis; Duodenal Ulcer; dissecting Aorta or leaking Abdomincal Aortic Aneurysm
Not an exhaustive list!