The overflow tank has pressure builted up even when the car is cold
Pressure should have backed off during the cool down, you could be looking at just a bad radiator cap, you can have it tested, or if it has never been replaced, just replace it.
You may also have a blown head gasket that's leaking fuel/air and of course, pressure into the cooling system. There a lot more pressure in the cylinders than in the cooling system.
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