Yes I think it needs to have somewhat like (at least) 4m of free space inbetween the copper node and anything else. When you've dug out the whole thing, it'll only start collapsing once you've mined out one bit so the other parts will "react" with it and -should- collapse. However, sadly it doesn't always work and as was said before; sometimes it's a bit buggy, maybe there's something somwhere inside that will still count as "ground" for the game.