I think it has to do with inacitivity. Maintaining a sitting position (in front of the computer, like I am right now, lol) leads to muscle imbalance where your back is tight but your stomach muscles are loose. If you continue this position daily for an extended period, then the muscle tightness will pull the joints out of alignment which leads to pain. I've had back pain for well over 3 years and didn't even find the cause of it until June 2011. Apparently, my left SI joint is jammed, leading to pelvic distortion. After numerous therapy in Canada including chiropractic adjustment, massage, and spinal decompression, I finally found some relief with medical acupuncture. I returned to my home in Yokohama, Japan in late March of this year and have been going to a acupuncture & moxibustion clinic since late May. I'm finally finding relief as my SI joint pain has diminished and my back has loosened up significantly.