Mostly likely because of either the worn out coolant reservoir cap or the over pressurized radiator. The radiator cap acts as valve letting cooland flow to and from the radiator to the reservoir tank. In addition, we also need to check the hoses and connectors as well.

Acura J series engines are notorious with weak coolant reserviour cap. So please take note of that. BMW and Mercedes in the meantime have very infamouse weak plastic coolant flange.