I got the idea while DMing a D&D game, and realizing that my players, upon seeing a poorly defended village will try to conquer it. They seem to be smart enough to not attack any highly populated towns, but small (orc) villages are fair game.
Anyway, a lot of the people who think too much into this fantasy game things state that kings will normally give land and stuff or marry off their daughters/sons to powerful adventurers in order to placate them and so they don't get killed. AD&D (D&D 2) actually says that many adventurers who reach level 9 actually have a fortress of their own, whether they steal it or whether they are given to them.
So basically, I see three motivations for this:
BloodthirstObvious. Adventurers are bored and powerful and want to kill something.. anything. More likely if you have a poor relationship with the adventurer's civilization.
GreedObvious. Adventurers know that a Dwarf Fortress, of all things, has plenty of -steel chain mail-, artifacts, and spare sapphires lying around. Yes, they are no better than kobold thieves in this case, but a hell lot more powerful and harder to satiate. Here, like in Evil Genius, the best strategy would be to leave a treasure chest full of valuables outside which they will loot, be happy about and leave. Also, it only works in the starting levels, when the fortress would probably be too weak - adventurers don't always loot things that they can't handle.
Power & ConquestObvious. A Dwarf Fortress is much more well defended than any human castle. But dwarves nobles are corrupt and don't deserve such power. So, an adventurer will see fit to 'liberate' a Dwarf Fortress, by killing the nobles and claiming the Fortress for himself. Or he might just kill everyone and place his own favored lackeys in there.
Advanced siege mechanics? Pfft. All you need is a champion with a big-ass sword, a weak mage who could melt axedwarfs with his mind, a rogue who could sneak into the castle and disable random levers as well as assassinating champion dwarves, and to top it off, a heavily armored cleric who heals and
resurrects the others.
Now that's fun