What is the reason why the arm can bend backwards?

If this situation can be reversed, it should be due to the relaxation of joint capsule and the injury of tendon and ligament. It is suggested to take a film first to see if there is any abnormality in the joint structure. If there is no abnormality, it is mainly caused by the pathological changes of joint capsule and surrounding ligaments and tendons. If necessary, it should be repaired and fixed to make it completely restored.