Before cooking, both the yeast and the Lactobacillus bacteria that are integral to the sourdough starter react with sugars in the dough to produce carbon dioxide gas. This is the rise that happens to the dough during proofing. This reaction stops during cooking though, because the heat inactivates the yeast and bacteria. During cooking, the rise you see is actually not a chemical reaction, but rather the expansion of the gas that is already there due to heat. The rise remains once the bread is finished cooking and cooled because the bread structure becomes rigid and stable during the cooking process.