In all of my forts, I never observed dwarven clumsiness: falling down stairs, falling over ledges, et cetera. (I have, of course, had my share of dwarves standing on tiles as they're mined out or deconstructed and silly things like that)
My recent project has been trying to carve out a fortress in a glacier. I've been replaying the initial couple months, and I've worked out how to my cistern to get water from a cave-in. I designate a couple tiles off the corner to let water out for dwarves to access, and a 7 Z-level stairwell to the surface.
For aesthetics, I designate all of the stone in the bottom to dumped into a garbage pile I create where the dwarves will be getting water. Suddenly, I find myself with two lightly injured dwarves and one very injured dwarf at the bottom of the stairs! My best guess is that one of them fell down the stairwell.
So I reload from the beginning and try again. This time I use a ramp instead of a stairwell to get down to the bottom. Designate the stone to be dumped, and I wind up with a dead dwarf this time, and I'm not really sure what happened: maybe got pushed off the ledge and fell one Z-level? (and was very unlucky, or maybe the stone she was carrying dropped on her)
Given that this has happened two for two, I'm suspecting there's something causal. What causes dwarves to be klutzy?
Are they automatically klutzes in freezing biomes? (both times I found the injured/dead dwarves in my stone layer, though)
Does crowding increase clumsiness? (both times my dwarves rushed into a relatively narrow area for dumping duty)
Are there any other factors I should be aware of?