ANSWER:
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.)