Low thread count along with low torque value (as a result of the lack of threads). What Ive experienced is the plug working loose over time for whatever reason. Probably expansion and contraction.
The reason I know this, is the day before mine blew, I heard a ticking while I was in a drive through. My window was open and I was hearing the sound reflect off the wall. It was too light to hear otherwise.
The ticking was the plug leaking as it had come loose. I changed the plugs on a friend's F150 with the same engine and it had a plug that was loose too. The coil pack boot was totally cooked.
The difference is his engine was a 2005, so it had the updated '8-thread' heads and it survived this until I replaced it.
Take a 4-thread head and loosen the plug a turn and you get a time bomb.
There is a place advising using a very high torque value on the spark plugs to prevent them from loosening, but, Ive checked and the stock value of 10ftlbs is about all you want to give a 4-thread head. Taking into account the strength of the cast aluminium.
An 8 thread head is much better in that respect, and 18-22ft-lbs is doable. So, I did tighten the plugs on the F150 to 18ft-lbs.
The repair is pretty much permanent. It converts it to an 8-thread hole, so barring spark plug installation error at some point, its permanent.