Some things to consider.
WCB is a fairly ductile matrial and as such, would be resistant to cracking. In the failure analysis, you should look at carbon content, verify proper heat treatment, and look for possible weld repair somewhere around the crack.
Also, there is a chance pipeline loads could have deformed the body and caused it to crack. Again, for a ductile material like WCB, when it is properly produced, would tend to yield and deform before cracking. But if the material is not processed correctly and is not ductile, it could crack if the valve body was used to jack mismatched pieces of piping together.
Other possible source of cracking is low temperature. If the material was not produced correctly (checmical composition, heat treament, and weld repair), the nil-ductility temperature could be up near ambient and the valve could crack when stressed. Doubt nil-ductility is the case, but should not be removed from the list of possible cuases until the inestigation data proves otherwise.
Best of luck in your root-cause analysis