I was at a buddys house and noticed his wheeler was apart, I asked him why and he said that he was moving if from the garage to the back yard and the spark plug blew out of the head and stripped the threads along with it. Just wondering why it happened. He didn't have an answer.

Jim

 

My best guess is the spark plug had at one point been over torqued and was "almost" stripped, it was just a matter of time before it let go completely!

 

Another common way is they cross thread it in.
This makes it feel really tight when only a couple threads are down and as soon as the pressure builds it blows out.

You should always use just your fingers to start the plug in until its finger tight. If that happens withing the first 2-3 revolutions its probably cross threaded, so you need to back it out and try again.