sparkplug coming out

I've talked to everybody I know and they've never experienced this, but now this Taurus has done the same thing twice.

One of the spark plugs comes out of the cylinder. I simply thread it back in and tighten it down and seems to operate normally. But it's alarming that this happens.

Does anybody have any clues out there?

Thanks

Does the plug go in easily or does it take a lot to turn it? If it takes a lot to turn, it could be bad threads on the head or the plug itself. Also, check the plug for damage. Make sure the piston is not hitting it. And of course, make sure it is to proper tourqe specs.