There are all kinds of reasons why that 904 could have came apart. At this point I am not sold on the idea that it rolled the Sprag making it happen. The driver would have had to keep his foot in it revving the engine into the 9k range to have enough RPM at the drum to have it explode. The drum would need to spin more than double the engine RPM for the Drum to be the reason it exploded. Correct me if I'm wrong but I thought a 904 drum can handle in the 20k RPM range before it exodes.