What causes air to come out of gas tank opening cap

There is a fuel vapor recovery system as part of the emissions equipment on modern vehicles. This captures fuel that has evaporated in your tank, which is the puff of "air" that you feel when you open your gas cap.

There is a charcoal canister that should capture the vapors from the tank and allow it to "breath" so that they do not build up in the tank and pressurize it.

You may have a clogged tube going to the charcoal canister or the charcoal canister itself may be clogged with something. The charcoal canister is usually a black plastic housing about the size of a shoebox. It may be mounted underneath the car or on the firewall in the engine compartment. To find it, just start tracing all the hoses from the fuel tank. You will need to check each one to make sure it's not clogged, and then test the charcoal canister to see if air will go into it.