Spice cake has sweet crispy top
German Apple Cake also responds like this. Must say that I've come to adore the crunchy top! :) But yes, you are right, it doesn't really happen if you bake it as a large cake. It's mostly just along the outer edge of the cake, in my case anyway.

My reasoning is that cupcakes contain less batter per surface area exposed to the oven... I think the oven just caramelizes the sugar in the batter. Think of meringue where the sugar isn't properly incorporated... you see little holes in the surface of the meringue. I'm not saying you should dissolve the sugar better in this cake recipe, but if the crunchy (slightly holey top) isn't desirable for you, try using caster sugar which dissolves easier.