How did the notion develop that a living thing killed via microwave would explode like an unpricked potato?
Because the microwaves would cause your internal fluids to boil, creating steam, and increasing your internal pressure until your skin could no longer contain it.
Since I haven’t personally tried microwaving any living animals (and if you have, I don’t want to know about it), this is purely speculation on my part, but it is what I would expect to happen based on what I know about basic physics.
If you’re exposed to non-lethal levels of microwave radiation, effects can range from none at all to cataracts, brain damage, and other fun things. In all cases, the heat caused by the microwaves is what does the damage; by denaturing the proteins in your body, IIRC. Same thing that happens when you slap a burger on the grille. I should note that bad effects are going to be maximized by microwave radiation that resonates with the water molecule; that’s how the microwave in your kitchen heats food up, by jostling around the water molecules.
A cubicle is just a padded cell without a door.
I would expect a living being to “crack”, but not explode. Just like Greg said, internal steam could reach the point that the skin could not contain it. However, I don’t think this would take very much pressure to split the skin, so the pressure would be released way before explosion time.
My guess as to why people think this is because they are putting unpricked potatoes in the microwave and making quick conclusions. When I was a kid, and one of my friends got the first microwave on the block, we entertained ourselves several times by exploding eggs. The novelty quickly wore off, because it was disgusting to clean up. 
On a semi-related note, try this cool experiment:
Get some grapes, and cut the grape almost all the way through the middle, but leave a little “hinge.” Now put the grape in the microwave, making sure to fold the grape open (like a book) and put the flat sides down. Turn on the microwave for 15 seconds max, and watch the sparks fly!
I’m not kidding about the sparks. An electrical current is induced into the grape halves, with one side negative and the other is positive. An electric arc will form between the grape halves, meaning there is a LOT of electricity, like thousands of volts.
BuckshotJones, yes that works & is well written about on the net.
Other much more spectacular things can be found searching for fun with microwave ovens.
IE get a toothpick, stick in something like clay, burn it first so it is black, put in microwave oven, watch a humongous display, far better than the fruit one.
Hmmmm… This would explain a lot of the cell phone people.
I used to put bugs in microwaves.
Cockroaches kinda swell and ooze.
But grasshoppers and crickets pop, especially if they’re about to molt.
http://www.joecartoon.com/reddot/gerbil.html
big ass SMILE
I would think that long before internal fluids boiled, the intestinal gasses and stuff would swell up in a major way and cause severe bloating/farting/explosion(?)
More microwave games. Take one of those AOL cd’s you get every week. Pop it in the microwave. Turn out the lights. Run the oven for 3 or 4 seconds and watch. Spitzin’, Sparkin’.
“The problem with the world is that everyone is a few drinks behind.” - Humphrey Bogart