RE: Why does this happen?
CEN did make new diff cups for the Magnum truck that are supposedly stronger. However, you may have to call Custom RC Cars or Toyhaus and have them order it as now one lists the part on their website. I emailed Custom and they stated their website was not up to date and they could get any of the Magnum parts.
On the dogbones and CV's, I think CEN used the same one on the new Magnum truck so we are still screwed there.