TMJ can be caused by;

1. Neuromuscular problems. The jaw uses surrounding muscles for movement. Misalignment of jaw joint, jaw, bones or teeth can cause facial muscles to work hard to compensate, with related pain.
2. Internal issues.  Relates to structure of joint capsule, containing bones, cartilage, fluids, nerves, and veins. Various problems can damage the soft cushioning cartilage disc of the joint, possibly leading to disc displacement, where the disc between the rounded end of the jawbone and the temporal bone is displaced.  The initiating problems include bite issues, teeth grinding (bruxism), joint misalignment and / or hormonal imbalances. The tell-tale symptom of this is a “click”, possibly as the disc clicks back into place after being temporarily moved.
3. Bone degeneration. Affects bone and disc’s support structure. Promoted by trauma, hormonal or other systemic issues.