Have no idea why this is happening. It happens with different flours and I am following recipes carefully.
The corollary question is "what gives loaves structure?". The gluten in the dough forms a network which acts as a skeleton to keep the holes (known as alveoli) from collapsing onto one another. If your gluten network is insufficient, this is what can happen.
Why did you not get more structure? There are a few likely possibilities. First, maybe the wheat didn't have enough gluten to begin with. Second, it is possible that you didn't knead it enough to develop the gluten to form that structure. Third, It is possible to overferment the dough and cause the structure to break down from enzymes within the wheat.
If you post your recipe and procedures we may be able to narrow it down some.
Ive had that happen with too much gluten. So more detail needed, please. Oven temp. Proofing temp? Mixer? How long a mix? Oven Hot air blower? Crumb shot, is it baked thru? Country?
I would say a lack of focus. Pan loaves will collapse inward on the side if they are not baked long enough but this is beyond that and into soufflé territory. It would make a nice bowl for beef stew.
after the bulk rise the dough needs more degassing before another rise or shaping to prevent one big bubble from forming and colapsing upon cooling, a lot like a pocket bread but in a tin. Pocket bread is made by shaping early so when gas bubbles form, they join together during the bake creating a hollow. If thats the case a longer bulk rise should help.