Steam PC version, Mouse/Keyboard, most recent update (4/21)
What was I doing to cause this?: I was merely returning to an area I had placed down items at.
So upon returning to an area where I've put stuff down assorted items (one-slot ore, two-slot research, etc), I find the once 'placed' items sometimes floating in the air like they've been bounced up. And no, the items were not packed together. Most have plenty of room to themselves.
I'm already under the assumption that the entire planet doesn't stay loaded, so with that in mind I figure only so many chunks stay loaded within range of a player. I'm guessing that when I get out of range of these items and come back, the reloading of the terrain and items causes them to sometimes jar out of place or pinch and/or jump off the ground?
But I could be totally wrong. I'm just taking a swing in the dark there.
On a side note:
Sometimes I notice rocks floating off ignoring gravity in a similar type of situation. No idea if it's connected but I figure I'd say something about it.