Curious re. exhaust Manifold cracks?

[Huck]

Just Curious! Have parted out several of these trucks and occasionally come upon a stock exhaust manifold where the cast iron has started to crack - in one case - totally thru the casting. Can kind of think it may have to do with heating and cooling especially if on the pass side due to water splash but most recent has been on a very stock original truck drivers side which still had the manifold cover on it. Are the casting contiminated from the factory and just explode slowly over time or ?? Anyone have a clue? Huck