This "explosion" thing and its timing has piqued my interest. Not too interested in who pays, but the actual mechanism of failure is certainly of interest.
Not sure how shops like the one in this incident go about flushing a cooling system. Do they use pressurizing equipment of some sort? What if the head gasket had a small, unknown leak and the flushing process filled the cylinder in question; then upon starting the engine that particular cylinder hydro-locked and expelled the spark plug stripping its threads in the process. Don't know. Just thinking.