Cause of spalled cones?
Time traveling here...

If that type of spalling occurs after several thousand miles, then that's most likely normal wear from the little bit of play vs road vibration, etc.

If that occurs within 200 miles, then it means your cone nuts are too tight.  Some people here say there should be no play, but that is incorrect.  There should be a tiny bit of play in the axle/bearings.  If you smack the axle, you should hear it tap lightly.  

If you have it too tight, you'll feel very little or zero play, but when you twist the axle by hand, you will feel it grab or catch very slightly.  Or, if you hold it by the axle ends and spin the wheel, you'll see it grab periodically (change in friction makes it twist a little in your hands).

That is the minor imperfections in the races and the bearings fighting each other.  There should be just a tiny bit more space than the largest imperfection combinations (for a micron or two of grease).  That means, most of the spacing will have just a tiny bit of play.

I find that every hub is a little different, but it usually takes 5-10 tweaks to get just the right amount of play when reassembling the wheel bearings/axle/cones/locknuts.