These Baker's "caves" or "tunnels" were just discussed in reference to bread making. I can't find the thread.
When I looked into it, I got all sorts of suggestions on The Fresh Loaf: Over-proofing, under-proofing, weak gluten development.....
What work for me, as I felt my gluten development was fine, was not being quite so gentle with the final loaf forming and getting a bit more of the gasses out. I hate to say it, but I think it is a combo of maybe weak gluten, and letting to many big bubbles form and maybe over-proofing.
I know pizza is not bread, but one could argue (though I will not) that this stuff is closer to bread than pizza.