There is a strand of muscle called Quadratus lumborum. It is attached to the last rib, Lumbar vertebrae, and iliac crest. If this muscle contracts and unable to relax, it will cause the spine in the lower back region to experience pressure and restricts the blood flow and causes pain.
There are also a significant portion of people who experience lower back pain due to hardened kidney. For this case, daily massage of the kidney would soften the kidney and reduce the lower back pain. While back in massage school, there was one student who had been experiencing severe lower back pain. We found out that his kidney was hard like stone. Then we helped him massage his kidney daily for about half a month. Then his lower back pain was gone permanently.