I immediately had that same question & here is how it was explained to me. If the plug wire or coil is going bad, it causes a mis-fire. The ECU then kicks up the voltage to the suspect cylinder to try & force spark through the plug & avoid damage to the engine. As the voltage spikes, this raises heat within the plug causing the core to expand & burst the insulator.
So, you can have good plugs in the engine, but if the wires or coil create too much resistance, the ECU still must raise the voltage to get adequate spark to that plug and poof, the plugs burst from the excess heat that is generated from an over-voltage condition.
Great to hear your car is running well. I hope it continues & serves you for a long time to come.
JimR
