I go with Hydro-lock. Hydrolocks exert tremendous pressures which is who there so many commercial uses for those systems. A shop once took apart one of my Caterpillar blade cylinders to replace the seals. The inexperianced
worker used 5 lbs. of air to push the rod/piston assembly out of the cyl. and did not restrain it. it shot about 20 feet across the shop hit the wall doing damage and came to rest undamaged it's self. He almost got fired over that stunt.
The part weighed about 75 lbs, and it only took 5 lbs. of air............
I suspect the break was in the weakest area of that engine and a lean condition may have assisted in the situation as I have had lean 4 strokes kick back violently!