My initial response was deleted, so I'll repost without the bit that was apparently offensive:

Copper will fall when it is completely disconnected. However, was is "connected" visually doesn't always match with what's connected as far as the game is concerned.

The easiest way to determine what is still connected to foundation is to use the mod "Ore Support." (I don't like it, but one of my friends loves it except in crypts, where it appears to cause a lot of lag.) It shows the bounding boxes of individual nodes and how/where the copper is still connected, and makes it easy to determine what exactly to strike to make the whole thing fall.

As for why fall damage is so high, I blame [insert figure that's been irrelevant for years].