Multiple myeloma, also known as plasma cell myeloma, is a cancer of plasma cells, a type of white blood cell normally responsible for producing antibodies. Often, no symptoms are noticed initially. When advanced, bone pain, bleeding, frequent infections, and anemia may occur. Complications may include amyloidosis. The cause is unknown."

Source : Wikipedia

Although the cause (or causes) are not yet determined, several 'risk factors' (i.e. conditions which might increase the risk of contracting the disease) are known -

The exact etiology of multiple myeloma (MM) remains unclear. However, frequent genetic alterations, particularly translocations involving chromosome 14, are commonly observed and likely contribute to disease development. Oncogenes such as NRAS, KRAS, and BRAF may also play a role in promoting plasma cell proliferation. Several risk factors, including obesity, alcohol consumption, and exposure to environmental factors like insecticides, organic solvents, Agent Orange, and radiation, have been associated with an increased risk of developing MM.

Source : Ministry of Health / Teaching Hospital Badulla / University of Colombo

Further info Cancer Research UK