This is the way I would see it. On the left is the belt running straight (ridge in red). Forces on a point on the edge of the belt add to zero. Now, on the right, If the belt wanders it gets deformed (think of the edge as a string) and the forces have a resultant which tries to restore the belt to the center. Note that the forces on the edge closest to the ridge are greater since the belt is more stretched on that side. (I have exaggerated the amount of deformation for clarity.)