This should probably be a bug report, which I'll submit next time that I see it happen...
While channeling out a large area of the surface, a cave in occurred. I'd thought that I'd been careful, but sometimes these things happen. Bury the victim move along...
Then, a cave in occurred another time, possibly at the same location (I hadn't look so closely at the first one.) A tree, perhaps? Plug the hole in the floor, rebuild the ramp, be glad nobody was injured...
And then, I'm pretty sure at a point in empty space that was complete devoid of anything, another cave-in occurred, punched a hole in the spot I'd filled in, an destroyed the same ramp that had been wrecked by the previous one. I paused and it looked like the cave in had begun from empty space, though it's hard to tell with all the debris. It definitely hit at the exact same spot as the previous cave-in, possibly two. Luckily no injuries, but rebuilding the surrounding area is rather tedious.
So, if this happens again (and I noticed that before it seemed to happen around the same date in-game) I will definitely make a bug report. But has anyone noticed something like this before? Is it already in the system?
For now, I imagine it's some kind of cosmic activity. Maybe in the future I can try to reinforce the spot and see if I can build enough stuff to keep it frmo punching a hole in the ground. Again.