That has been a running theory but not accepted by all. Just as some believe gut bacteria plays a role in all autoimmune disease.
Source: Have neen diagnosed with MS for 18 years and have been diagnosed with RA for 25 years. There isn't to my knowledge any one thing all agree on as the main cause. One of my great neurologists I've had said to me early on in my diagnosis, the more they learn about it the less they seem to know. Since new research often contradicts prior widely held beliefs.