So it would seem that this is a common occurrence?
Common enough. If it ain't da trogs, it's da antmen or whatever.
It's like JG said - if you have an exposed chasm, they come pouring out if you're too close.
Too close to an exposed magma pipe/pool is even more fun.
Most common critters an untrained dorf can usually wrestle down, even those singly, but not a wave of them.
In the future, your options are...
1) Once you embark, look around. Use < u > to see what wildlife there is, and where.
2) Don't accept embarks with chasms that end in the middle (this is weak - just be careful)
3) bring along an armed dwarf (or more than one) of some sort*
4) laugh it off like we do and start over
(* see "Ambusher" in the wiki)
In extremis, any miner and a pick add to the fun. You're not defenseless, just have to look at what's around. Sometimes the sphincter factor is higher than others is all.
I bring a dorf with one level (each) of ambusher, axe, architect, and some leader skills, plus high armour use. Comes with leather armor, a crossbow and steel bolts (the axe is up to me.) Make him my (untrained) wood-cutter/plant gatherer. With an axe in his hands and someone else unarmoured but armed with that crossbow they can stop most unpleasantness. (Not sure about a half dozen trogs...)