If one falls on the butt, and the butt hits the ground, there is one possibility that would bring on mid back pain. The assumption here is that on impact, the person literally sat on the ground. At the instant of impact, the upper torso and spine would most likely be leaning forward or curved forward. The one possibility that the sudden and hard impact of sitting on the ground would affect the mid back would be if the tailbone hit the ground with full force. In such a case, the buttocks would be less endowed and the tailbone would have been less protected. This would indicate that the buttocks would be relatively flat, there is slight protrusion of the tailbone, and the person would be relatively skinny.
With such physical characteristics, it would be possible that the tailbone hit the ground and the impact sent a shock wave up the spine, and as the muscles on the two sides of the spine tighten to protect the spine, tightening of the muscles brought on a misalignment of a spinal disc in the mid back. In a sense, the tightening of the spinal muscles “froze” the misalignment. This would bring on pain in the middle back.