I've never had a tantrum spiral finish off a fortress, but the last one I had came entirely too close. After a goblin ambush killed 2 or 3 ad hoc militia members the dwarves gathered at the meeting hall which they'd never really left since it was designated to remember those fallen. Then, they departed peacefully going back to their jobs and food/booze stocks. A little bit later one of them decided that sleeping in the bunk beds wasn't good enough, or maybe it was the merely incredible rather than uberroyal dining room that put him over the edge. He tantrums, and whines and complains, and annoys some others who do the same.
Then, they all calm down.
Then, one random dwarf concludes that he's utterly sick of the same food and throws his plump helmet at someone I suppose. A dwarf who took that as murderin' words. More tantrums, this time leading to the spiral proper. As the spiral continues any attempts to resolve it fail. I resort to drastic measures, locking the doors, everyone inside stays inside to die, everyone outside is dispersed enough to survive. Naturally the goblins know this is the time to ambush, three times in a few moments. So now, score is all inside consigned to die in the darkness, those outside probably for the goblins or consigned to gibber madly for the rest of their days. This having taken about 2 seasons, the liason in the fortress and some traders or guards gives in to the madness now as well. They're tough, several berserk dwarves failed to take him down.
As the population winds down, when the blood ceases to run down the walls, 3 survivors outside 1 in the courtyard (locked from both inside and outside) and 2 outside. 2 survivors inside, one wounded to the point of probably never moving again, and the berserk liason. Carefully cautiously herding the remaining outside survivors in to the courtyard, I manage to lose another one. It's beginning to feel like a bad level of lemmings. The living, sane badly wounded survivor dies of thirst, leaving 2 outside and 1 berserker inside with goblins beyond the courtyard but no food or such in the courtyard. A little bit of shuffling and circle running, two dwarves manage to get in while at the same time in a bit of pure brilliance tricking the berserk liason outside, locking the door behind them.
Of the 2, remaining out of 112(!), one promptly calms down from running circles around the berserk liason, and begins to gibber madly, and refuses to eat.
The sole survivor, put herself a bed and some pretty baubles in a nice room, farmed a little bit locked the doors and waited, and waited. Alone in the dark, trying to spruce the place up for immigrants she prayed for, cleaning blood off the walls and avoiding the stinking carcasses of 100 some dead dwarves and many animals.
The best part of course, the *only* bright side to this story, immigrants arrived rather promptly although the first wave was too few in number to clean the place up. The survivor is offically my most epic dwarf as she survived probably everything that spiral had to offer, goblins liasons other dwarves madness starvation. Nearly 10 years later she lives, and has been reelected as mayor perpetually ever since, becoming legendary in several skills and miraculously without any permanent injuries from the ordeal.
The closest I'd ever come to losing a fort to tantrum spiral before that is one that went from 60 or so down to 2 or 3, and when I saw the last walking survivor in the barracks, covered in mud blood and vomit, I couldn't bear to torture them anymore and abandoned that fortress.