Green banana peels contain a plant hormone called ethylene. This is a gaseous chemical that is responsible for helping ripen the banana to yellow. At the same time, a few different natural acids are also being produced that add balance to the ripening process and make the banana sweet.
When you put a banana in the fridge, the acid production slows down because of the cold temperature and, as a result, the ripening process slows down. Yet ethylene production continues to escape, eventually breaking down the cell walls of the peel until it becomes black.