From what I see it's because he ground so much material out of the top of the arm to compensate for the initial tie rod contact and then went further when he installed the larger diameter tie rod. He basically caused this failure.Those big caster plates likely helped the arms bind up laterally sooner than they would have otherwise. Could have contributed to the failure.
And it's been shown that under severe use the stock brackets can tear apart. The plates could have likely stopped that from happening.