Sourdough English Muffin ?

I made the Sourdough English Muffin recipe posted earlier by PMcCool. While I was cooking them I noticed they developed a really huge bubble. When I flipped them over I gently flattened them. When you split them they have a huge cranny in them! or is that a nook? I was wondering what I might have done to cause this. It did not happen with the last batch. Externally they look great and taste great too. Also made 2 loaves of Sourdough..the fall weather really makes me want to bake, bake, bake!