Tongue is partly due to the blood trapped in the head, as Jon says - as well as closing the windpipe,the noose normally closes the jugular veins (no blood can't leave the head), but a slightly loose noose won't block the arteries, which are deeper and stronger (so blood is still going into the head). But also, depending quite where the noose is on the front of the neck, it can force the tongue either out, or back into the throat.