What makes that happen? Dough can ferment and even fall, yet it will rise again after shaping.
Is it that the yeast is being redistributed and exposed to more food? Or what?
Yeasted doughs generally rise three, maybe four times before collapsing. The dough collapses because the gluten network can no longer withstand the pressure of the gas building up inside of it. If that is so, there could be more food for the yeast to continue to consume and expel gas, you just wouldn't know it is there.
In reality I think its a combination of both the elasticity of the gluten network and exposure to fresh food sources for the yeast.
When you degas and shape, any tension in the gluten network from the pressure of building gas bubbles disappeared, as those bubbles are popped. In addition the yeast is being redistributed as a product of the shaping.