Same thing happened to me in a 2001 lightning. I was showing the truck off and with it in park just reved it a few times and it blew the plug right out of the head.

The thing was under warrenty and when we had it at a ford dealer the service tech told me it was fairly common and that it was going to fixed at no cost to us. They actually put a new head on and recommended a different plug for it. I sold the truck before it ever needed a new set of plugs so I never had any follow up to the different plug thing.
Truck had 26,xxx miles on it and was about 2 years old at the time. All stock no pulleys or anything else.