I have treated hundreds of patients with back pain and I have had a lot of back pain myself. I'd say I have a pretty good perspective on the problem. Not to take away from anybody else's theories, but most garden-variety back pain is mechanical in origin. Deconditioned muscles, wear and tear on supporting ligaments, facet syndrome, congenital anomalies, idiopathic or acquired scoliosis, degenerative change or osteoarthritis, is just a partial list of potential causes.
The spine is a single, synchronous, functional unit, but with hundreds of moving parts, some joints as tiny as the nail of your little finger. I doubt such a complex mechanical structure fails because of emotional lability.