Timing issue.... Although I don't quite understand how the primer could have been in line with the hammer/firing pin but yet it appears to have hit the left side of the barrel shank.
I didn't see any flame come out the end of the barrel, I bet the shot before this one was a squib. The pics they show don't show all the way down the barrel, I still bet there is a squib somewhere down there, especially with the barrel split like that.