Why does multiple myeloma cause bone pain?

Multiple myeloma grows in cortical bone and bone marrow which causes generalized thinning or damage to local bone, which is known as Osteoporosis. 

The main cause of bone pain in multiple myeloma is due to soft spots in the bone or osteolytic lesions. On an x-ray it appears as holes. These spots in the bone are painful.

Any bone can be affected but most common bones which gets affected are ribs or back.