once something starts burning, how does it stop itself from burning?
"Self-extinguishing" material stops burning, because it's already burnt part doesn't shed or become porous, blocking oxygen from the non-burnt part underneath it, so the fire is smothered. Think of a fire in your fireplace. When it's about to go out, you take the poker, and knock the burnt part of the wood off, exposing the unburnt part to oxygen, and the fire increases. In self-extinguishing material, the burnt material tends to stay on and block the oxygen, and, there's no one with a poker, knocking the burnt part off.