Any number of factors will have caused that & I would have thought there would have been indicators prior to the catastrophe.
Sparkplug readings should have shown something that may have enabled a fix, but if, say, an engine is running very well, sounds 'fit', the tendency is to leave it alone. Truth is, there needs to be that extra safety margin. In this instance, I suspect jetting, which might easily be caused by altitude changes....
It'd be interesting if you add information to see the sparkplug, underside of the piston & the first internal part of the exhaust. As well as ignition, carburation, fuel, oil etc
If you achieved as good a seal between cylinder & exhaust as you did with head to barrel, I will be impressed
Even more so if a head gasket was used....not that will ever get me to use one again in the conventional 'sandwich' context