What kind of things can cause a Diff bearing to self destruct?

I was pulling my trans out this weekend to change FD for my brother to take to Driver's school and found the passenger side bearing destroyed. 10 of the 18 or so ball bearing were missing completly and only about 2 of the ones that were still there were anygood at all. The other ones were ground down and I can't find the ones. The magnet has plenty of metal stacked up on it. The gears seem to be good so far. I'll be interested to see if my posi is trashed.

I want to make sure I don't make the same mistake when I put my new trans back together. I can only assume it was an install issue since I had just built that trans with new bearing about 6 months ago with only about 4 weekends on it.

Thanks