Why may the relief valve on the diesel engine cylinder head lift?

If a liquid (cooling water, fuel, lub oil) accumulated in the cylinder, the piston on it's upward movement develops extremely high hydraulic pressure as liquid is incompressible. The relief valve set to open (lift) if cylinder pressure exceeds a safe limit and activated in order to save the engine components from damage.

The other common reason is the oil accumulated in the cylinder f.e. due to excessive pre-lubrication. It is always advised to turn the engine by turning gear (or slow turning) prior starting to check for liquid accumulation and to ventilate possible accumulated oil vapors.