Once you see how things work, it's just common sense how parts fail.
If you pulled the head on the old motor, you probably would find at least one or more broken intake valve heads... once the head pops off, the stem can fall into the cylinder and will end up jamming into things... it's even possible for parts to migrate to another cylinder in extreme conditions when an engine tangles up it's parts.
Most likely the pieces of the intake valves got caught in the exhaust valves... got all smashed up by the pistons, got locked up and stopped the exhaust cam.... hence shearing off the bolts as the weakest link between the cam chain and the sprocket bolts.
Edit: 2/23/18, after some discussion with you, I do not think that this applies to you, your failure is due to something else.... disassembly of your cylinder head will give a lot more info in your case.________________________________________________________________________________________________________
Many years ago (1999), I had customer's ZX9R blow it's motor on my dyno after I installed one of my jet kits...
The cause of it was the locator tab on one of the clamps that holds the carb to the head
see attached photo that I took from ebay todayOne of the clamps popped off when I took the carbs off, and during the handling of the clamp when re-installing it onto the intake boot the locator tap popped off (weak spot weld) and fell into the #4 cylinder unknown to me at the time.
When I started the bike it was just quiet as a mouse... I revved it a few times and took it around the building to my dyno room at my old shop... rolled up onto the dyno, strapped it down.. got everything set up, and proceeded to make the pulls... on the 2nd pull it blew at 12000 rpm.
Apparently what happened was the little tab embedded itself in the piston crown... when it came out, it got caught in one of the exhaust valves, the head came off the valve and broke the other 3 valves in that cylinder... the head split... the valve head ended up in another cylinder... There was so much damage to the rods (bent) and the crank/clutch gear teeth were bent due to the force when the head split. What was most amazing, the cam chain didn't break !!
My insurance was very good about paying for the parts and labor at the time, and the customer was fixed up and running fine 3 weeks later... he wasn't even mad at me and is still a customer till this day.
That was the only direct experience with a blown motor that I have ever had... other than an Indian 2 stroke that broke a piston on a friends dirtbike when I was about 13.
Ivan