Yeah, it's an (or several) old, old issue(s) with the generator not always checking if a tile is supported when generated, and then deciding to check after you start playing. I never actually saw it happen with trees, but I have witnessed it happen with frozen-over ocean tiles. There was a big issue a while back, before the new site maps were made, where the ground would have huge vertical chasms all throughout, with many sections that werent supported. Players would spawn in, the game would realize the village theyre in or adjacent to had no supporting structure, lag the fuck out trying to process all those cave-ins at once, and you'd immediately afterwards fall to your death in the caverns below.
I'm guessing the entire world didn't collapse from underneath me. I was still on the surface, ish, more of a single-tile pit as cave entrances will often have a few of, trees and the dome itself still present. I just fell and did the splits way too hard.
Today, I played around some more and stumbled across a group of undead alligator snapping turtle corpse bandits and their human hammerman caretaker. They're all fairly cool. Snapping turtles, not snapping turtle men, it should be noted. All they equipment they would have had was in a bunch of piles nearby.
aaaand after giving some of the turtles' equipment to my elf companion and then trying to give his wood shield back, he refuses it, because it is a rude bauble fit only for my kind.
You were just fucking holding the thing five seconds ago you fuck