Possible balance issue, or just unlucky?

If the crank and rotating assembly was out of balance and caused these 2 issues, then I expect you would have felt the vibrations long before the drag strip visit. Do you have any evidence or feel for an imbalance besides these 2 faiiures?

Without other evidence of imbalance, I would be inclined to attribute the causes for these 2 failures to things like pulley misalignment or overspeed belts, and the threads failing in the TC block, or similar issues.