I think its a bit of the dough that sticks to the cast iron and burns. And then once there's a bit on there, the next one will catch a bit, burn a bit more, etc, etc, etc.

I have tried making pancakes in my big bessemer pan, and it took me AGES playing with the heat. Frustrated the you-know-what's out of me!

As for the BBQ, not sure the Man would let me near one. lol