Why would a spark plug blow out?

The lady friend drives a 99 civic. The she was driving though a hilly area and said the car was acting sluggish and the CEL came on and then went way. After that, the car drove fine.

Today, coming back through that same area the car did the same thing, but didn't get better. A few miles latter she described the sound the car was making as that of a lawn mower and the CEL stayed on.

When she got home, I opened the hood and could see one plug wire laying out and no plug to be found. I don't see any damage to the threads in the block. Anyone have a guess as to what the underlying cause might be?

 

Somebody forgot to tighten it "correctly". There is no way it would fall out otherwise.

 

Pulled the codes and it was showing a misfire on all 4 cylinders. I figure when it blew the plug out, and that cylinder was getting 0 compression, that they others all started losing timing.

I looked at the piston head and didn't see any damage, so I am pretty sure it didn't detonate or physically touch the plug. So, at least I have that going for me.

After changing the plug and the plug wires, I restarted it and it sounds fine. No codes are popping up. So, I guess that plug just worked its way loose and then finally was pushed out by the compression pressure.

 

I bet it just was forgotten to torque and just hand threaded in

 

I had an '82 Corolla that used to launch them like Scud missles every few months. The threads in the head were worn or something. Loud bang, run like shit, pull over. The plug was usually still attached to the wire. I'd tighten it back up and drive away.