two things right off the top of my head. 1, not enough anti-freeze in the coolant mixture and the boiling point is too low. The anti-freeze raises the boiling point of the coolant. When you shut your car off the water pump stops pumping water but the engine is still hot and the coolant will continue to increase in temperature for a short time until the engine starts to cool.
2, your coolant resivor cap is not sealing or is not allowing the system to pressurize. The pressurization of the cooling system raises the boiling point of the coolant mixture as well. Lack of presure could allow the coolant to boil when the engine is turned off when its up to temperature and the coolant stops circulating.
