I'm betting that this is a delayed failure of a cracked rim after it got hit by something like a pothole or a curb (my father destroyed a wheel in similar, albeit immediate, fashion with similar damage pattern after running into an unseen curb).

From what I can see, the impact started a crack on the outer edge of the rim on the break with the jagged edge. The crack propogated under fatigue stress to the spoke hole, then turned and propogated along the tip of the "V" in the V-section rim to the next spoke hole. The crack then turned at the hole (because the hole is reinforced) and propogated toward the opposite braking surface. Failure was when the crack finally reached the other edge of the rim. (See attached picture)

This is definitely not due to air pressure or tire size (because the initial failure is a radial crack on the rim, rather than circumferential), and for similar reasons, not caused by a worn braking surface (besides, the rims only have 500 miles on them); these two failures often go hand in hand besides. It would have failed either due to a hit on a pothole, curb, or other object known to taco wheels; or it was due to a manufacturing defect or mishandling to cause the initial crack.