It's helpful to think about how aerosol cans work in order to understand why they carry a "do not incinerate" warning label. Basically the aerosol can contains two substances - the product which is a liquid at room temperature and the other is the propellant which is a compressed gas. When you depress the valve at the top of an aerosol can, it opens and the propellant expands because it is a gas. The propellant expansion forces the product out the nozzle. So let's imagine this pressurized aerosol can being burned, or incinerated. The can will be heated to very high temperatures. The propellant will expand and expand until the can ruptures or explodes. Many propellants are flammable. Additionally, the product in the can may also be flammable leading to a fiery explosion or flash fire. Obviously this is dangerous and that's why the cans carry "do not incinerate" labels.