Better living through chemistry. Copper is a very reactive metal and loves elements like oxygen and sulfur (and other things) which tend to float around in the atmosphere. Plus the coin may have been stored somewhere in an environment that could include this stuff. Add the possibility of heat increasing the reaction rates, and you can have myriad of possibilities. There are just too many ways for this to occur to even hazard a guess
It's not a copper coin. There's a very thin copper plating on the zinc coins. Those zinc cents tone easily, and that color will change. Nice rotation though.
