Engine damage

[CumminsPower98]

I looked at a relatives '94 after being pulled from truck and head/pan taken off this week and have some questions. #4 cylinder died with broken valves. Both valve heads came off and the exhaust was broken in 3 places and of course bent the push rod on that one and ruined the cylinder and piston of course. All 6 exhaust valves hit the pistons leaving heavy imprints. The exhaust valves have the HD springs for e-brake. One gear case bolt was in the pan. The one that fell out was the one near the oil pump. I didn't see any damage from this other then the bolt was chewed up some. The question is what caused the piston/valve contact on only the exhaust valves. It seems like if they floated all the valves would be affected.

The truck has seen many owners and is turned up. It pulls loads and is not well taken care of and has over 200k I think but could be wrong as it might be under 200. I did an HG (external leak near t-stat) on it last summer and adjusted the valves a few months later with no problems and it ran fine for some time.

Ok, that's pretty much the info on the truck. My thoughts are over rev on a down grade with load maybe but then why just the exhaust valves with HD springs? I don't see how the fallen case bolt could have caused this. Could the cam key have sheared somehow and retarted the cam? Could that bolt have shocked the gear train some way and caused the cam gear to move? The cam has not been pulled or engine turned to check timing. I'm just looking for ideas as to what could have caused this.