In an aerosol can a gas is filled under a very high pressure. Due to the high pressure a part of the gas is converted to a liquid. When the button of the aerosol can is depressed, there is an opening created and the pressurized gas inside the can is expelled out of it. As this reduces the level of the pressure inside, a part of the liquid is converted to gas till the equilibrium is attained.
There is no relation between the gases above the liquid inside the aerosol can helping the liquid to come out. The whole process is due to the highly pressurized gas filled inside.