That's what I'm thinking too, coolant rushing in and bam, there goes the piston rod since it can't compress the water.
They had just spent 3k fixing oil leaks on that engine too.
Usually the head gaskets don't fail like that though. They'll leak coolant and oil from the right rear corner, and occasionally on the right center. Occasionally you'll find a broken head bolt on the right center. I can't recall ever seeing coolant enter a cylinder.
Sometimes the graphite rings between adjacent cylinders will disintegrate allowing combustion gases to enter the cooling system. This just blows coolant out of the overflow bottle.
Has this truck had particularly loud cold start chatter, or a cylinder nailing while going down the road? It could have been a bad injector overfueling and damaging a piston.
Another infrequent but devastating event is when the valves are never adjusted at all. These valves get looser over time, not tighter. Eventually so much play develops that the crosshead is able to jump up over the tip of the valves. It lands on top of the spring retainer. The next time the rocker presses down, the valve locks jump out of the retainers and it instantly drops one or both valves. I've never actually seen this, but I've heard about a couple of incidents. The valves are supposed to be adjusted every 50k. 10 times more miles than that without an adjustment is begging for a disaster.
Edit: Did it kick a rod out of the block? For curiosity's sake, if the pan comes off, get someone to look up at the bottom of the pistons. Have the wrist pin bushings started to drift out of the rods? They shouldn't be visible when looking up at them, but if you see one or more that might indicate a number of severe piston overheats, either from coolant loss or overfueling. I've rebuilt a couple of well abused 2007 engines with wrist pin bushings (which are pressed into place) loosen up. This is often paired with half of the piston crown having melted away due to overheated pistons being overfueled by leaking injectors.
Edit 2: Actually, we have an example of this in the shop right now. 2007 338. Terrible injectors were ignored forever, and coolant leaks never fixed. The owners only spent a dime on the truck when it failed to move.
Number 1 piston scuffed...badly. It had been chronically overfueled and nothing was ever done about it. This continued until half of the crown melted away and disappeared out of the exhaust pipe.
http://i.imgur.com/eD3sKJq.jpg
As the piston started welding itself to the cylinder wall, the rod started to bend. The rod has a compound bend. It was close to breaking. The piston, attached to a rod that was bent in two different directions, started to crack. The rod physically shorter because of the bends, allowed the crankshaft counterweights to hit the bottom of the piston. I don't know how long it was driven in this state, but it was driven at least 40 miles to our shop like this.
http://i.imgur.com/5SB364J.jpg
This is abuse, plain and simple. A small problem was allowed to develop into a larger one, and the larger problems were also ignored.
