Originally Posted by
smd4
I would think the force exerted would tend to pull the axle towards the front of the dropout, along the slot. What caused it to force the bottom part of the dropout (I'm guessing), down, vertically?
Usually it happens as a result of the chain jamming and pulling on the derailleur. This could have happened under a previous owner, who straightened the damage. A subsequent hard shift may have been enough to cause the failure in that previously weakened spot.