The causes of hard bumps on the back include common warts, boils, dermatofibromas, scars, and so on. 1. Common warts: If the patient has common warts, they may appear as benign growths on the skin due to the infection of the skin by the human papilloma virus, thus causing a hard bump on the back. 2. Boils: If the patient is a boil formed by hair follicles and surrounding tissues, due to bacterial infections and other infections that cause swelling of the skin, the symptoms of a hard bag on the back can also occur. 3. Dermatofibroma: If the patient is suffering from dermatofibroma, it may appear as a hard bag on the back due to excessive proliferation of fibrous tissues, thus protruding from the back. 4. Scars: If the patient has a traumatic injury on the back, the fibrous tissue proliferation after the trauma, the phenomenon of hard bags can appear on the back. Finally, the phenomenon of a long hard bag on the back, please consult a doctor in time.