Sue, the Wisconsin DNR link was very enlightening, since it goes beyond the basic "what is" answer for frost cracks and sunscald. Thanks!
It's unlikely that the crack you show, Kabby, is a frost crack. The physical force that causes frost cracks would have pulled the bark much farther apart. More likely it is caused by some defect or infection under the bark and the normal pull/pressure forces present in any tree. (That's why the crack has not widened.)
Chelle, I don't want to poo poo your effort to find possible explanations: few people take the time, or even know how to think outside the box (or even inside the box, for that matter). You deserve kudos for that. However, I am going to shoot down your shading theory of the epicormic growth.
Bark that is that old and thick would be sufficient protection from frost cracking on its own. Even if one were to assign an intelligence to the tee to "hurry up and protect the trunk with shade", an evolutionary path to back up such a theory is lacking. And it probably doesn't get that cold in Alabama, anyway(?). More likely, the sudden difference in light trigger adventitious growth from under the bark layer, and/or possibly some other stress as mentioned earlier. The rush of growth, relative to the canopy growth is due to the presence of several factors, including the convergence of optimal conditions: lots of light with a relatively protected and more favorable environment, an unlimited supply of water and nutrients and the faster, vigorous growth that comes with the juvenile growth that they are. (Normal canopy growth is always mature growth.)
But Chelle, if sunscald were a possibility (on thin bark) then your shading theory could have merit.