The whole molded casing just popped. There are 6 or so large pieces and many other small ones, most of which fell onto the ground at the launch site and were lost. The motor was in an aeropack 29-38 adapter, and the top of the casing looks like it was cleanly cut off right at the top of the adapter. The pressure drop must have been sudden enough that all the grains were snuffed out. Fortunately there was no damage to the rocket or pad.