Sounds to me like a loyalty cascade.
Thanks. I had seen references to tantrum spirals and loyalty cascades, but I hadn't seen that page in the wiki. I don't think I did any of the causes listed, but the job cancellation spam symptom sounds right.
I actually wrote up my post about two weeks ago when this originally happened, but it took a few days to get my account verified, so I'm just now posting it. What happened next was interesting.
The precipitous drop in population trickled down to a few skirmishes until I had just 49 dwarves left, maybe 20 of whom were in the hospital, and most of the rest were children, when a vile force of darkness arrived.... Early in the history of the fortress, as I was channeling from the surface to irrigate a subterranean farm plot, there was a cave-in. I never thought to patch it up, but two goblins managed to enter the fort through it. They went into the hospital and methodically murdered all of the recovering dwarves in their beds. Pretty soon every one in the fortress was dead, except for two dwarven children. One was hiding in a bedroom as I watched one goblin go from door to door, checking each room, but for some reason skipped right over the one with the child (I didn't think to lock the door). Shortly after the siege broke, some migrants arrived. There was no other way into the fortress, since the drawbridge was up (and I didn't leave a lever under the doormat!), so I assigned "build a floor" over the caved-in area, then channeled it out in such a way that at least one dwarf would fall in and be able to "Pull the Lever."
Long story short, I'm still playing my first fortress because two children managed to survive. (Well, one of them went insane and died of dehydration or something.) And I'm back up to a population of 140!