Why do brownies made from commercial mixes form a flaky top?
If I could offer a possibility..... When you bake a loaf of bread, the surface, exposed part of the loaf has a different crust than the interior (even when baked in a pan). This is due to the carmelization of the sugars on the surface. I suspect that this is the case here as well.

I get the same effect baking a mix or a scratch recipe. I've always assumed it is due to the carmelization of the surface sugars coupled with a bit of dehydration due to direct heat exposure....