I saw an Aviation Week article being e-mailed around that said that it wasn't mechanical, or even pilot error really. It was supposedly a thermal anomaly right above the compound; for some reason the air above the compound (because it's hardpacked dirt?) was 15O F warmer than ambient temps all around the area, and the helicopter abruptly lost lift when they flew into it and the pilot couldn't get it back in time before they pancaked.
15oF translates to 3% less dense air (not to mention the warmer air probably would have caused an updraft).
Are helicopters that sensitive to air density? Wonder how close it was to its weight limit?