One reason for a totally random sprawling dungeon (weak--but the best I can do).
In a magical environment, castles are not the strategic defensive positions that they were in medieval Europe. It is simply too easy to go over or through the walls.
As a result, many leaders desiring a stronghold have gone down. It is hard to fly over or otherwise gain a bird's-eye view of the defensive position. This makes it harder to teleport into, and virtually impossible to simply fly into a well-defended courtyard.
An enemy can still magically tunnel into the place--but if the stronghold is not built symetrically and takes a few other precautions, some of this can be defended against (at least a little).