Mid back pain can be caused by overuse, muscle strain or injury to muscles, ligaments and discs that support your spine.
Poor posture especially an exaggerated kyphosis (curved spine) can add extra pressure and strain.
Osteoarthritis, osteoporosis (reduced bone matrix), fractured vertebrae, herniated discs and more rarely a thoracic spinal stenosis can all exacerbate and irritate nerves around this region of the mid / upper back.
Risk factors include: slumping with sitting or standing, getting jolted in a car, getting hit in the back or lifting something too heavy.
In rare cases, upper and middle back pain may be caused by other problems such as gallbladder disease, cancer or an infection.