There are a few good FB boards for MM and one that is only for MM caregivers. My guess is that there is similar for colon cancer. They don’t seem to really know the cause for MM. But there are automatic VA benefits for those diagnosed with MM who were exposed to Agent Orange.
Very high incidence of it with first responders for 9-11.
There is an abnormal chromosomal variation.
Most of the time it effects the bone. My husband’s didn’t effect his very much. It hit his kidney’s.

Anecdotally there seems to be a link with anemia. I personally have noticed a number of people with MM that have had Lyme. DH has had Lyme at least 3 times or chronic.
Both his aunt’s and his grandmother had pernicious anemia. Supposedly his grandmother had MM but that was in the 70s so his MM specialist doesn’t consider that a sound diagnosis as the disease categories have evolved a lot in 45 years. She also lived 4 years after diagnosis which would have been really long for that time frame and the treatments available. Her symptoms don’t really match MM either. It was a blood cancer but beyond that who knows.

As far as I know there is no genetic link for MM.