Spark Plug blasts through engine cover?!?!?

Hi All - I've spent a couple of hours looking for this topic, and as far as I can tell, there is none. I'm sorry for the length.
After dealing with some intermittent misfires (DTC P0300), and getting those resolved by replacing the valve cover gasket, spark plugs and the spark plug wires, my 2006 Aveo drove great for about a month. Then, suddenly, the check engine light (MIL) came on. I drove a couple of days with it on, because there didn't seem to be any problems with starting, acceleration, anything. After a couple of days, I plugged in the (cheap) scanner and got a DTC of P0420, with a pending DTC P0300. Still, nothing seemed to be wrong at all - it drove as well as ever. So, I took my time researching what to do about those codes. That stopped Friday when, on my way to work (16 miles of a 26 mile commute) I heard a loud pop, and then the car drove like the exhaust system was disconnected and the spark plugs were only occasionally firing. After I got it towed home (thank you GEICO), I began to try and find out what happened. Since the exhaust seemed very much indicated I jacked the car up to get a better look. To get more light I opened the hood. What a shock! The engine cover was blasted into three pieces breaking apart right over the #1 spark plug. Most of the cover was still there and still fastened. I found the other pieces in a couple of different places in the engine compartment and then looked at the plugs. The #1 plug was laying in the channel (on top of the other wires) with the spark plug wire still attached! It had blown out of the chamber and was only slowed when it hit the engine cover. The bottom threads on the plug had some silver metal in them, which I can only assume means the plug took some of the threads in the chamber with it. There were also hundreds of tiny pieces of that metal (aluminum?) in the channel atop the valve cover.
And that is where I am now, trying to figure out:
- what happened?
- how do I fix it?
- how do I keep it from happening again

One fairly likely cause for this is an overtorqued plug, so the question is who did the work? Heli-coil is the typical repair for a competent mechanic or machine shop.

either what avguy said and the plug what overtorqued.. or the old plug took out the threads with it, or the new plug wasn't tightened into the hole properly.

luckily, it might be as easy as putting the plug back in.. with a hell-coil most likely.

Thanks for the ideas, guys. I was the mechanic and this was the third time in a year that I've changed the plugs and torqued them all the same (216 in. lbs.). This was the only time one "blew up" on me. Anyway, I was able to get a new plug seated and torqued (I actually did them all), and reset the valve cover and v c gasket (no real reason except to remove a broken bolt from the valve cover; replaced the bolt and the gasket). Fired it up and it purred like a kitten. Didn't have time to do anything else, so I'll be driving it to work tomorrow without further testing and with fingers and toes crossed. Update tomorrow...