The best fortress fall I can remember came when I breached the caverns for the first time. One moment I'm admiring the newly-exposed fungal trees, and the next moment I notice some blind cave ogres are in my fort's unguarded halls, chasing down my civilians. I hadn't set up a military yet. After a few deaths and lots of injuries, I finally got all three of the cave ogres trapped in some cages and put them away.
Human merchants came along a while later and I decided to barter them the cave ogres. But when a dwarf is told to haul a container (such as a cage) to the trade depot, he'll sift through the contents and throw out anything that can't be traded (such as a wild cave ogre).
So, yeah... the ogres were loose again.
While this defeat was entirely my fault, the best part was still quite random. Somewhere in the spiral of death, a miner went beserk. At last, someone armed with a sharp metal tool had become brave enough to stand up to the blind cave ogre menace! Surely he'd kill every dwarf in his way, but he was strong and armed and my fort's denizens would have their revenge on the ogres once my beserk miner set eyes on them!
...No, wait, he's ignoring them. He's not even hitting any dwarves. Wait, he's running toward the fort entrance. Now he's headed toward the depot. Oh, I see now. He wanted to kill one of the human merchants. Sure, okay.
The four caravan guards reacted sensibly enough to a dwarf sinking his mining pick into a merchant's brain-- by putting down the madman and then rushing into the fortress, crossbows blazing. A glorious bloodbath ensued! Glorious for the humans, at least. They were nice enough to kill the cave ogres before conquering my fort, so that was neighborly of them.