If it had, I do not believe I could have held the plane. I tried to analyze what occurred. Perhaps there was some gas in the piston chamber, the valves and piston were such that combustion could occur and the hot plug fired the gas. The engine on this plane is a Magnum four stroke XL52. Connecting a ni-starter on an inverted engine is sometimes difficult.
Many pilots just lift the plane while still in front of it.
So, on that plane I still have to lift it, but I do it from behind. My new policy for inverted engines is to always install an extension for the glow plug. I shudder to think how many times I have helped my grandson start his P51 warbird with a YS-91 engine. He picks it up from behind while I connect the ni-starter in front. I hope this helps to avoid an injury.