Could it be hot fuel? Say it had 40 or 50% nitro in it. It is somewhat clear that detonation occurred. That means lean mixture. Just the right circumstances (fuel air mixture just right for it to occur) and detonation at the right time and spot. Where it broke may be the weakest point and the cylinder just could not take the overpressure. Very similar to hydraulic lock failure. No matter, that engine is toast now. Things happen!