Have the cap and the cooling system pressure tested. A weak cap can cause this. Exhaust gases in the system will definitely cause this. Now for the science, as Selfrich mentioned the pressurized system boils at a higher point. Your car should have a 13lb. cap so theoretically the coolant should start to boil at 230 degrees and when it boils it creates pressure. When you shut your car off the coolant quits circulating and superheats to a certain extent. If it goes above 230 degrees, the cap lets the excess pressure off along with some coolant. Being too full can increase this. Letting the car idle for a minute or so before shutting off can help but isn't always a cure. This is why vehicles from the last few decades have overflow reservoirs. Now worst-case scenario is exhaust gases causing this, such as cracked head or a leaking head gasket. Sorry for the long reply but this explains it.