Damn, I need to start playing this game again. My last fort died with my old computer, and it was going strong too. I had a fully functioning military plenty of dorms, food, etc...
My last fort that actually died due to in-game events was raided by goblins, my military was busy boozing it up so when the goblin menace arrived it was too late. The goblins pretty much blocked off all the floors. They'd patrol the stairs which would scare the dwarves away from escaping. They all slowly died from dehydration and starvation. With a Dwarven Child of about 3 years of age, being crowned king of the mountain home before he slowly starved and froze to death.
Kinda like in my first town in Banished. Everyone starved and froze to death... only that was because they were a buncha lazy dumbasses.