Best guess so far for the cause of the helo crash is "vortex ring"- where, when descending in the hover, the air that goes down through the rotors cannot disperse (trapped by a confined space, for example) and flows up and around the outside, then back down again, so the helicopter rotor is trying to fly in a column of air that is falling much faster. Think of it like trying to actually gain height on a Stairmaster and you've got the picture...
The only way to avoid a crash when this happens is to fly forward out of the falling column of air- but if you're trying to descend into a confined space like a compound with high walls you're stuffed!
This is of course only my guess- it fits the reported facts, but it could be completely wrong.
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