If I were to guess I would say they probably had poor grade...

If I were to guess I would say they probably had poor grade control over the shallower oxide zone and greater data density/drilling in the fresher material beneath this.  They probably took the continuity of shallow grade for granted in a zone where weathering processes can either concentrate or deplete grade in a local area.  In this case they may have suffered greater depletion zones than anticipated.  In saying that though, I think issues with grade will become significantly less of an issue with increased mining depth. 

This is why I'm happy to hold my stock, despite the sphincter tightening fall from where is was 6 months or so ago.  I've even picked up a large number of cheapies and am more than happy to hold.