Okay, so here's what I think happened: I chose the wrong color as a counter-enamel.
I've been creating cloisonnes, and in this instance, you need a base enamel (on the front) in which to sink the cloisonne wires and you also need a counter enamel (on the back) to prevent cracking.
Cracking occurs because the glass enamel and the metal cool at different rates, making the metal warp as it cools. Now, if you have too much enamel on one side and not enough counter weight on the other side, the metal will warp too much in one direction, cracking the glass and, in some cases, popping the glass right off the metal.
Now, second point of interest, warm colored enamels are unstable and red enamels are down right temperamental. "Unstable," in enamel-speak, means that a color could fade over several kiln-firings or that it will oxidize when applied to certain metals; for instance transparent red and purple enamels turn brown on silver and transparent oranges and browns will go cloudy on copper.
So, of course I chose an opaque red as my counter enamel. I mean, ouch. You live and you learn... and I had to learn this one the hard way. Over several firings, it must have oxidized under the surface and just de-fused. Not pretty. Not fun. Don't ever use Thompson Enamel 1860, "Orient Red" as a counter enamel... you're just asking for a world of hurt.