Why would gas leak out of lawn mower cap area?

I assume you are referring to the cap on the fuel tank. The cap has a vent hole so the tank will not create a vaccum as it delivers duel to the engine. If the tank is half-full or more, vibration caused by using the implement will casue the fuel to "splash" within the fuel tank and also "splash" out trhough the vent hole. Check the inside of the fuel cap to see if the diaphragm is worn or missing and replace the cap if necessary.