What could cause the PCM to spontaneously drain a battery and then start smoking when powered back up?

Sunday the truck is fine. Saturday the battery is stone dead. I hook up the smart charger and it says "Error". I double check connections and it still says "Error". I figure I'm smarter than a smart charger so I hook up the old stand-by jump starter and set to it "charge". Hood light comes on, I open the door and all the interior lights come on then I see smoke coming from the battery corner! I immediately unhook the charger.

So now I'm thinking maybe the smart charger is smarter than me and my battery is bad. So I swap batteries with a good deep cycle. As soon as I start to connect the posts smoke starts again from the PCM. Do they just finally give up & go bad? I'm familiar with failing ones that cause loose of features but to apparently just short circuit itself?

I've gone over and over in my mind what I could have done/changed over the weekend (I had a productive, until this, truck love weekend prepping for Indy). Nothing electrical and nothing that could have pinched or cut a wire. What gives???