I was doing some removal work to facilitate replacing the bent cassette and did a little checking of the cogs on the cassette. I found several of them had burrs on the drive side of the tooth that looked recent (shiny). I'm thinking this was a clue about what caused my bike to blow up, but I'm not sure what exactly it means. The burrs are a circular arc on the side of the tooth that bears the drive force from the chain, and look like a tremendous amount of force was applied to the tooth via the chain, but the post mortem after the explosion looked like the initial failure happened somewhere in the return side of the drivetrain. So, wha' happen'd?
 
Opus