The sprag is not the real issue. It's the drum. The sprag failing creates the issue where the drum fails. If the drum was built better, the sprag failure would be moot. 904's don't have this issue. There have been explosions of billet druims in 727's, they are rare though.
Generally the sprag ring doesn't spin, the rollers and springs get shocked/smashed up and all detent is lost to hold the rear drum. Then the front drum can go on it's merry way above about 12K rpm, reach critical mass and boom.
Poor burnout technique, letting the tires bite while still in the throttle in first gear out of a burnout, breaking an axle/driveshaft, rear gear can roll the sprag making the "safety"/design feature of the sprag disappear. A low band apply valve body can help, not a guarantee to save a sprag.
