That crack is from rust. I have also seen stock arms crack in the same location from bad handling (as I suspect - monkeys doing the R&R on the bushings). I have also seen bent stock arms (easy to spot is you sight through the bushings).
the entire point wasn't to decide why the stock one cracked. it was to point out that even if your running stock stuff that you need to still check it from time to time.
it was in reply to this comment.
I don`t care how good a weldor you are, In real life, welds fatigue, something I would inspect frequently, and if they crack from inside out you`ll find out the hard way.