Here’s a case where hearthlings are glued to each other by just standing around. No items were needed to jam them.
They did not separate later? Collisions happen between entities and blocks, not between entities.
I restarted the game and it is fine now.
It looks like you hit upon a recognised, but not particularly well understood (at least AFAIK) bug which causes hearthlings to sometimes simply freeze in place.
The fact that they’ve all done it in the same location is very interesting though… if it happens again, I’m sure the Devs would appreciate if you’d upload a copy of your save game so they can check whether something about that corner is causing them to get stuck.