What might be happeniong is that there is some pressure leakage when the car is turned off and when you start it it burns off. You should look for leaks of coolant especially near the exhaust manifold before you start the car. Hopefully it is just a bad radiator cap and a leak in the hose to the overflow reserviour or it might be overflowing the resoerviour. Make sure you check your fluid /coolant level when vehicle is cool.
If you have not serviced the anti-freeze in the last two years it is very important that you get it flsuhed out and filled as it is no longer anti freeze and can cause problems.