[Walt Fricke]

Your picture of the necked down stud strikes me as a classic representation of a fastener whose plastic limit has been exceeded from too much tension. Since the amount of material doesn't change, making the rod permanently longer has to make it narrower somewhere.

So I don't think twisting had anything to do with this, at least not twisting to remove the barrel nut. That's not going to elongate the stud. Sure can snap it, of course, and likely where it is narrower.