Were these bearings defective?
I've also never seen a bearing ball fail like that.  In Jahaziel's answer to this question:

When do bearings need to be replaced? 

he notes that in Honduras where he lives they sometimes could only get cheap Chinese bearings that "used to 'peel off' like an onion".  Perhaps you got some of those?  See also the response of "Mike" in this thread [1], who had a similar (albeit worse) experience with cheap bearings.  

Good properly tightened bearings should last for many thousands of miles/kms, and usually races tend to show the damage more clearly before the balls do.  I've never actually over-tightened a hub and tried to ride on it to see how it fails when it seizes (and I'm not saying you did that, but it's the only other explanation I can think of other than cheap bearings), but based on both the SKF paper linked above and this paper [2] and this analysis [3] it seems like ball bearings delaminating or fragmenting is not the most common failure mode, because none of them discuss it.  Particularly in [3] you can see that the balls in seized bearings  polished off a contact patch against the race rather than disintegrating.

[1] http://www.bikeforums.net/archive/index.php/t-23340.html

[2] http://www.elongo.com/pdfs/BearingFailures990519.pdf 

[3] http://met-tech.com/failure-analysis-of-ball-bearings.html