>>63844697
The mixed solid bipropellant of large solid rocket motors can explode for a lot of reasons. Among them imperfections in manufacture. They are tricky to make and naturally push danger bounds. Because that system was firing it is what I expect in this case. Somewhere in that booster motor propellant that should have burned (deflagrated) producing steady gas and thrust instead exploded (detonated) at the speed of shockwave through the material. Badaboom, related.