Any thoughts on why my sure-grip did this?

That is what happens when the two small pinion gears seize to the shaft. It causes the shaft to turn in the case breaking the .250 dowel pin. what causes this starts by the cones bottoming out in the cases and therefore spins the pinion gears more than was intended by design. Yours is the worst case I've seen of damage to the case. It was ran hard long after the cones quit holding in the case. Those two small pinions are impossible to find. The shaft can be replaced with a shaft from an open rear, but the gears? Good luck finding them. I have several cases I could built SG's out of if I could source the small gears.