Weak mixture is the cause.
The spark acting like a spark eroder minus the dielectric, which is the incoming gas. It takes place over quite a few miles before the centre of the piston, which is the unsupported weakest point, breaks through.The crown of the piston will appear clean, as if it had been gritblasted, all traces of carbon gone. Sometimes you might get a whiskered plug, but it`s not carbon, it`s molten aluminium. Ask me how I know. Also I have several pistons (not mine) of various makes showing the same result. I`ve mentioned to Ernie, that it will be one of three things that would make this happen. Air leak manifold, air leak head joint, or more commonly, blocked carburettor.
One piston of Minivins, when it was bought to me for investigation, did not have a hole, but was starting to collapse on the crown. I mentioned this and asked if anything had happened to cause this, the reply was that a while back he had a split rubber manifold, that had been replaced, since then miles had been put on but the piston did not hole. But it was replaced.