MS fatigue is variously described and the causes variously attributed. The only generally agreed upon thought is that it’s an “MS thing”.
For me I think muscle stiffness is most likely at the root. Stiffness that causes poor sleep over night. Poor sleep that leads to typical fatigue during the day.
I think that that daytime fatigue is also the source of a lot of balance and mobility issues – not being awake enough for adequate concentration on controlling and coordinating muscles. I think the fatigue (and general low body energy levels) is also at the source of the post big feed “coma” that often hits hard.