That guy extended his left hand up enough to get his left index finger on the front of the trigger guard! That is a good way to get your hand flash burned to start with. I wonder if somehow he pushed on the cylinder release with that odd grip?

Oddly he didn't seem surprised by what happened?? The camera guy didn't even flinch?
Hmmm, the hard to cock hammer could have meant another round projecting out the case and a double ignition of both powders?
Karl

After looking and thinking more it appears that something hit the forcing cone edge to the left to cause that barrel to spin around like that rather than being propelled forward. Hitting the forcing cone could have split the barrel for sure. Makes me think more about the cylinder not being timed up correctly or as mentioned the cylinder release inadvertently pushed?