Wow! Talk about a failure.
OK, a few things come to mind and I am sure that I am probably wrong.
After all I'm a monkey with a gun when it comes to mechanical things. But I do know something about physics.
1) What type of fuel were you using? If a higher octane than normal the more "blast" you would have. That compression could have blown a seal and forced the piston into it's current position.
2) Just a bad seal, it gave. Although I think you would have had some sort of warning sign before it got that far.
3) Somehow more fuel than normal was forced into the chamber, the ignition spark hits that higher than normal fuel ratio and BLAM. Although this is more commonly called a backfire or misfire.
4) Bad parts to begin with (and most likely the answer). How old was the scoot? What model? How many miles? All that matters but if I were to bet money on it I would say you had a hairline crack that expanded over time. Of course, again, there should have been some sign to that.
Anyhow...monkey with a gun so who the hell knows.