Why does coolant leak out your car after you have put it into park but not while you are driving?

Why does coolant leak out your car after you have put it into park but not while you are driving?
their can be a couple of reasons for this, 1- while the car is in gear the drive train is loaded and the assembly is twisted(suspect weak/broken motor mounts) flexing coolant hoses. So there is a possible coolant leak with anything connected to coolant/heater hoses. Check radiator hoses, clamps, and the metal/plastic tubes that come off radiator. Also on plastic tank radiators, the rubber "o" ring gets hard over time and produces leaks. 2- You could have a semi clogged radiator, reason being that after being driven for a period of time and put in park that the heat from engine is can not be dissipated causing the coolant to boil over. 3- If your car has an electric cooling fan it could not be working, so while driving the coolant is circulating well, but when stopped their is no cool air passing through the radiator causing an over heating problem.