The overall condition of the cones argues against rust being a factor. Go look at railroad tracks to understand why. If rust were a factor, we'd see a polished (or worn) area where the balls ran, with unremoved rust to either side.
While classic fretting is a possibility, that's a slow process, and is more common in stationary bearings (headsets). This is called a fatigue failure, which is a mechanism similar to the cracking of ice on a lake (as I described earlier). The moving local stress of the balls depresses the surface, which rebounds behind the ball. The constant movement, especially if not well supported from below flakes the meal. You can see a similar process in real time if you use a rolling pin to roll some pie crust, let it dry a bit, then try rolling some more.
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