Why would my basic whole wheat bread turn out with a crack along one side? I could understand two sides if I'd done something wrong, but why one side?
You haven't done anything wrong.
The "oven spring" (last additional rise in the oven before your leavening dies) is simply too much for the forming crust on your bread to contain. So the weakest point in the crust "gives" and spreads the forming crust to allow the expansion. You can't stop it, but you can control it--even use it as a decorative element. That's why many loaves get a "slash" before they go in the oven.