4 possibilities I can think of.
1) The threads were bad in the head.
2) you cross threaded the plug
3) you forgot to torque the plug down.
4) water was in the cylinder, it over pressurized and the plug was was the weak point.
hopefully it was one of the first 3, if the fourth, hopefully no damage to the rod.
But you know what fixing it properly means.... off comes the head. I once fixed a sparkplug hole in a AL head on a 4 cylinder turbo mazda without removing it. did my best to keep chips from going into the clyinder and then blew it out, vacumed and finally turned the engine over with the plug removed so anything that remained shot out the plug hole. engine ran fine for a good long time after that. (used a helicoil on it)