Most likely it started when one dwarf started tantruming and fell off the edge, or perhaps a construction accident, setting off the rest. It wasn't a sparring accident, my barracks are still on the land. It didn't help that I jumped the gun a bit, trying to move my dwarfs up there when all I really had was a main 3x20 (unwalled) walkway, with paths leading off to nearby rooms containing the bedrooms and dining areas.
I've since moved them back to my minimalist land fortress and let them calm down a bit; I'm not really interested in keeping them too happy though, as I've already accomplished most of my goals from this fort and I'll probably never touch it again once DF2010 is released. I'll take what I've learned from this one and build SeaFort 2.0, the bigger and better and hopefully less deadly fortress based on my current one. Also, I'll make sure mermaids are present before building it.
At any rate, the last thing that happened was a failed attempt to move my caged fire imp. I'm not entirely sure what happened, but I told my dwarfs to bring my imp to the trade depot (I was curious how much it was worth and too lazy to check the wiki) and they somehow interpreted it as take the cage to the depot and not the imp. So of course they let it out of its cage, right in the middle of my fort. I got my military to run over and chop it up before it did any real damage, but I almost let it run loose for a while to punish the stupids for letting it out in the first place.
Next major thing I do, as soon as DF2010 is released, is to get all my dwarfs up on the sea platform and deconstruct the narrow walkway holding it up. The entire platform is currently supported by a one-tile-wide walkway, so it shouldn't take long.