car was running fine then...bad news. looked under the hood, the #1 plug wire was flopping around and the spark plug was M.I.A.. how could this happen?

 

It sounds to me like the plug wasn't tightened down enough and the compression from the cylinder kept pushing it out until it unthreaded the whole way and pushed it and your wire/boot right out of the tube. I have never heard of this happening, but it's the only explanation I can think of.