The pain from the sciatic nerve usually comes from a misalignment of the spinal bones in your lower back due to variety of reasons but those bones shift, compressing the nerve. It may also be due to a muscle spasm, a pelvic bone shift or a combination of some or all of the above. The cause of the bones shifting out of place varies, but it may be due to injury or a bone spur, the piriformis muscle, a pelvic misalignment or a combination of all of them. The cause of the bone misalignment may very, but could be due to a disc herniation, disc injury or accident, a bone spur or just years of poor posture.