Back pain behind the left scapula can be caused by trauma to the back of the shoulder resulting in soft tissue injury or fracture of the back of the shoulder, or it can be caused by myofasciitis of the back of the shoulder due to chronic strain, or it can be caused by organic lesions of the heart, such as myocardial infarction or angina pectoris which may involve the back of the shoulder after the development of pain in the precordial region.
Therefore, once the back of the left scapula pain occurs, you need to seek prompt medical attention to identify the cause.