As I see it
1. Everyone who was present said that the sound of the round going off was significantly louder than the rest of the shots being fired.
2. The shell in question must have had a large load of a high energy powder such as Bullseye to blow the gun. It probably was purposely overloaded and not an accidental overload. I do not think that a double load of a powder such as Reddot would have caused the burst chamber.
3. The shooter said that he was firing factory ammo. He is credible. I do not think that anyone would be crazy enough to knowingly load or fire such a shell.
In conclusion I feel the shell in question was purposely overloaded prior to the shooter obtaining the ammo.