Wow! I have limited breadbaking experience, and am sure someone else will explain it, but in the meantime, you've got yourself the perfect bread bowl for entertaining.
Make that hot dip with the spinach and cheese, and invite friends for March Madness! If you could work out how to replicate the hollow loaf, you might be able to make some money on the recipe if you sold it to Panera or another bakery chain.
The most likely result was there was an air pocket to begin with before the dough was placed into the oven, i.e., When your husband rolled the dough he did not roll it tightly and there was a seam inside. This is often done on Artisan breads and specific types of European Style Breads and Rolls.
I think it is from: too much yeast, long rise time, not enough kneeding after the punch down
Could be one of the above or a combination
I like bread like that, except when I want slices
Couldn't have been too much yeast as a natural levain like sourdough, doesn't work like a dry yeast which can be "over measured". That said, it looks like it was rolled into shape rather than punched into shape, leaving a rather large pocket in the center. As it rose and later expanded in the oven, this pocket enlarged, pushing the dough around it to the outside where it baked/set quickly enough to hold it's shape. The void then, simply stayed. Next time, don't roll it into the loaf shape, pat and "coax" it and make sure there are as few air bubbles as possible. Very active starter will cause all kinds of interesting aberrations. Good luck.
yah, kinda like a rose and the way my husband described my step mother-beautiful&fragrant on the outside but watch out for those thorns