I thought this piece was dry enough and that my firing cycle long enough to dry it thoroughly through and through... but in the very first rise of the kiln up to 200 degrees, when water begins to boil, there were some pockets of wet deep within, and that little bit of steam caused all this mess.
The bad news was that about $20+ worth of underglazes were layered onto this sucker and it wasted a bunch of energy blowing it up and then turning it into gravel.
The good news, if there is any, is that this was a piece that I wasn't so sure about. I'm saved several hours of sanding and mess and another firing. And there's one less big, heavy piece to haul up to the show--tho I'm SURE it would've sold if I'd had it there. It was still an interesting piece to work on and learn from during the forming process.
The longer I'm a potter the more I learn what silly mistakes I can make.