It isn't uncommon for them to crack,but most exhaust manifolds around here that fail are due to rust,they flake off layer after layer until they are very thin,then get hot,and one good dousing when you drive through a deep puddle can cause them to crack when they get thin..
Most crack where yours did..they expand and contract quite a bit--often you cant bolt one up again once it is removed without hogging the bolt holes out they shrink so much..they even sell "spreader" tools to make installing them easier,but cast iron doesn't like being "stretched"..usually cracks instead..
I have heated a manifold up in my wood stove a few times and used welding gloves to hold it while I get the bolts in !..
Some of the aftermarket ones aren't good quality,they too can have a short life..ones sold under the Dorman name are usually pretty good--new ones with a "no name" brand often sold at salvage yards are often cheap china crap that crack easily ,I have seen more than one crack while being installed,and one that had cracks right out of the box..look them over good before you leave the parts store !..
