I've lost two so far I think.
First one was due to a tantrum spiral that was due to a goblin siege. Shutting the drawbridge to keep the gobgobs out (military? What's that?) turned out to be a bad idea. Largely because the only water source was outside. Whoops.
Second one was due to a tantrum spiral that was due to a goblin siege, in this case i had to shut the drawbridge with too many dwarves still wandering around outside. They all ate it, as did the outdoor pasture. I had indoor plumbing this time, but screwed up my pressure calculations and flooded the hospital and dining room.
The siege I got rid of with traps, but finding a dozen dead farm animals, a bunch of cats and dogs, some children, a baby or five, and a dozen dead dwarves soured the population on sanity. I abandoned that one before it had a chance to completely grind down to nothing, it was too depressing.
Third fortress is still going, it has weathered no fewer than two tantrum spirals, a couple seriously massive gobgob/troll attacks, and has so many piles of equipment from dead dwarves/goblins that my FPS is suffering. I need to find some magma to fry the stuff with, or just stack it under an atom smasher. Nobody seems to be willing to haul it though, which makes things difficult.
I expect this one'll die from boredom, or a titan.
I voted goblin siege, as that was the initial cause in both cases.
Actually there was a third I forgot about, I didn't play it very long.
Had a wonderful embark site right near a cliff, had a river, peaceful, tons of metals, huge amount of iron ore and coal, and a glorious 10 z waterfall on the surface.
Dorfs kept turning up missing. After loosing six of the seven+first immigrant wave, I had a sudden thought.
Sudden thought: Dwarves don't swim well.
Sure enough, six corpses at the bottom of the waterfall. That explained that, a bit late.