Why would an exhaust header crack?

if it's freshly hot and parked and you run cold water over it

 

hot,cold,hot,cold,hot,cold....... throw in vibration and age, you get the picture.

 

You forgot one more hot,cold

 

If the header is not flush with the side of the head when the nuts or bolts are tightened, the header can twist (torque) when cinched up. Such twisting can lead to a crack over time with infinite hot and cold cycles.

 

Also, if the rest of the exhaust system is not properly supported, the headers could be helping to support the exhaust, and these extra loads on the headers can accelerate cracking.

 

Excess heat, poor support in the rest of the system, bad design, previous owner abuse

 

Yes but he threw a bonus vibration in. That usually scores me points

 

Running lean can also cause this.