We'll never know for sure why the thing crashed. It does look like he came in too hot and settled onto the wall, but really - who knows.
Maybe Bin Laden himself shot the engines out with his Makarov - they just won't tell us, because that would make him look like a Terrorist Hero to his fellow jihadists.
Much better to blame the crash on VRS, created by the compound walls.. that's less embarrassing than "mechanical malfunction".
And to the general public, some vortex-thing doesn't even sound like "pilot error"!