Did you use thread-cutting oil on the die when you were cutting the new threads? Some types of stainless can really gall when put under stress, and if it did that it would "grab" the cutting surfaces of the die and possibly cause them to break. Thread-cutting lubricant prevents that from happening.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.