There is only one way over-pressure can happen in a cooling system on any water cooled motor, and that is by compression getting into the cooling system from a source. Those sources can be, a leaking head gasket (most common) or a cracked head (why they magaflux test heads for hairline cracks), or a cracked block (least common). The water pump cannot cause over-pressure, sorry, it is just not possible. Most water pumps only pump about 5 -7 psig at normal engine RPM's; not near enough to cause over-pressure.
Overheating can cause over-pressure when water flashes to steam. When this happens it will lift the safety over-pressure spring in the radiator cap and blow off...

I concur with the others; looks to me like your head gasket was leaking?

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