Cause? Well it probably wasn't due to excessive maintenance...........
My bet is a sticky valve hanging open, the piston hits it bending the stem, mayhem ensues.
This is not rare. Hint; when waking up a long dormant motor, pull the valve covers hit the valve stems and cylinders with PB blaster or other penetrating lube, kick the *itch over a bunch of times with the plugs out, set valves, cam chain etc. kick it some more before attempting a start.
Here's a shot of a similar event.
There were piston chunks out in the air cleaner.
Because I'm a slacker the engine became a light weight mock up shell.
Someone bet a guy he couldn't crush a 6 pack of empty beer cans with his forehead. He won. Oh, he died the next morning of a brain hemorrhage, Oops.
Carry on.