What would cause the top of the water bottle to the radiator on a Kubota mule to keep popping off causing the engine to run hot?

The only way for enough pressure to build in the coolant bottle to blow the top off of it is for pressure from the combustion chamber to enter the cooling system through a leaking head gasket or a crack in the cylinder head or a crack in the cylinder wall releasing compressed air fuel mixture into the water jacket. You will notice bubbles in the water bottle soon after starting the engine and you might detect the smell of fuel coming from the bubbles. That would be a dead give away since fuel should never be present in the cooling system under normal conditions.