I would say environmental damage or just a harsh life, don't take this the wrong way but it wasn't even worth the coin flip and staples. The reason I say this is because you can still see the copper coating but keep on looking they are out there.
looks a lot like some i put on top of a wood furnace as a kid, the zinc would melt but the copper would retain its shape. upon cooling the copper would take on a black tone and the zinc would re-solidify. if i cut them in half i could melt the zinc out of the copper.