The current follows the least resistance. On a spark plug the current jumps between the gap in the electrodes. When the plug is out of the cylinder the easiest path is between the electrodes. But when you put the plug in the cylinder, the resistance in the gap is increased due to compression. So the current found a place with smaller resistance to travel. There was some defect, crack or something in the plug somewhere to redirect the current somewhere else. Probably too all for the naked eye. Hope that makes sense. I'm not real good at description.

The current follows the least resistance. On a spark plug the current jumps between the gap in the electrodes. When the plug is out of the cylinder the easiest path is between the electrodes. But when you put the plug in the cylinder, the resistance in the gap is increased due to compression. So the current found a place with smaller resistance to travel. There was some defect, crack or something in the plug somewhere to redirect the current somewhere else. Probably too small for the naked eye. Hope that makes sense. I'm not real good at description.