Cornbread that rises quickly when you put it in the oven, then collapses and gets really dense as it cools, is most likely overmixed. If you overmix your batter, the baking powder will start to rise too early and will cause the cornbread to puff up, then collapse. Leaving your dough a little lumpy will help the cornbread to rise slowly. This is an example of overmixed cornbread: