Why does the sea level fall during glacial periods?

The only way for sea levels to fall is if the water goes somewhere else. Where else could it go? It could remain on land, but only if it was frozen. That is the answer. If precipitation always falls as snow and hail and the climate is cold enough, like Antarctica, then the frozen water builds steadily up on land. More and more water comes out of the oceans and then falls frozen onto land, where it stays there.