It's been a complaint of people for as long as I can recall.
I also know that I've been using tall stair-stacks, despite this threat, for most of my playing life and haven't had such an accident as I'm led to believe can happen.
Maybe I've just never had a fight happen on a stairwell, the (momentary) loser going tumbling to the ground-that-isn't there, etc and doing an impression of a slinky. Though my last fort latest version had some cavern-creature sneak in and definitely was in a knock-down fight with several dwarves sent to help the one that got ambushed. No-one (even the creature) fell down stairs, despite a stairwell being the scene of the various (across Zs) encounters. Maybe just luck.
Thus I know it's a thing, but don't know how much of one. Your scaffolding-dismantling deaths sound like a confluence of actions and places to stand to do them that interacted badly with yet other actions. But that's a guess. If it is, though, then better micromanagement might have helped avert such disasters.
(I also don't use stairway-scaffolding, having other solutions to various building configurations, so haven't tried mass-removal a layer at a time. I'd suggest, though, make it every-other top-level stair-scaffold, then start with the remainder after enough clearance of the first lot. Might avert a repeat of this, if I've imagined the situatkon right. Only mildly more awkward to do.)