Why did the excavated ancients collapse at the sight of light?

Oxygen is a particularly active chemical element, and it must do everything possible to react with things it touches (especially organic matter) as soon as it catches the opportunity. As a result, what it embraces (mostly on the epidermis, because the deep matter is dense and difficult to immerse) deteriorates and becomes other matter.

Some things have been buried underground for thousands of years, and due to special reasons, their space is sealed and isolated from the air (that is, oxygen), so the human body and colored terracotta warriors and horses can still maintain their original colors in this case (as long as the environment remains unchanged, it will not be a problem for 10 thousand years). Once dug out and exposed to the air, the obvious change only takes a few minutes, and it will be completely finished in an hour or two. Light is not the main reason, and the fading process of light is an extremely slow reaction.