[jakeru]

When an engine is overheating and continues to be driven, especially when there is enough vapor in the system that "normal" coolant circulation by the water pump ceases, the temperaratures of the engine head will continue to increase, and rate of boiling of the coolant will accelerate. The coolant liquid or vapors can only escape out the coolant cap/pressure relief vent so quickly, so "runaway" overheating could allow excessive pressure to build up (well above what the pressure relief cap would start to vent at) which would then cause the system to start to relieve this pressure at the next weakest link, whereever that is (in this case, apparently it was somewhere around the right radiator).