There is no one reason for your middle back pain. It depends on the work type you are doing and your life styles. If you have a work of sitting in front of computer for long time and your chair is not comfortable then you can have the disease. If you are quite a lazy person and spent most of your time lying on the bed or sofa watching TV, then you can have the middle back problem. You can also get the middle back problem by not sleeping in right position or may be you fell down from some height or gets slip so you can encounter the middle back problems.  If you have middle back problem then you reason can be different but the common thing is having middle back problem.