I don't think this is something to be concerned about. It only overflows for a few seconds, then stops, right? As the engine cools while parking the car overnight, the coolant in the overflow tank gets sucked back into the top of the radiator. And as this happens it fills the radiator right to the brim. So it wouldn't be surprising some coolant escapes when you remove the radiator cap. More likely to happen in certain situations I expect, depends on the temp of the coolant in the radiator, and if the car is parked on a slope or not.
If still concerned, you could do a quick check for a head gasket problem. Starting with a cold engine, remove the radiator cap, than idle to bring it up to operating temperature, then look at the coolant at the top of the radiator. See any bubbles coming up? Rev the engine to 2500 or so, check for bubbles again. No bubbles, unlikely to have a head gasket problem.