that is odd.. at least what is visible in last frames .. the chopper is quite stable and not falling down... maybe they hit something else not visible
Even though the speed of descent may not look that high from the distance the vid is taken, it's actually quite a high speed. Also note the hilly and wooded terrain and you can guess that it probably landed on a slope and hit a few trees in the process.
The Mi-24 is a quite heavy bastard and it needs a realitively high forward speed to be able to perform a safe auto-rotation landing. And that's on flat terrain without object.
Bottom line; if you get shot down in a chopper, 90% chance you're fvcked
All the US (and other, but mainly Saudi) aid for the Mujaheddin was channelled through the Pakistani ISI (Inter-Service Intelligence; the Pakistani equivalent of CIA but with stronger military connections). The later was taking decision who in Afghanistan is going to get the biggest piece of cake - and supplying most of aid foremost to Hekmatyar's and other groups known for their religious zeal.
Taliban were established by the ISI in 1992-1993, when Hekmayar disappointed the Pakistanis - and at least four years after the US aid was stopped.
Correspondingly, even if there was a large number of Arab volunteers in Afghanistan as of 1988-1989, (when the CIA stopped supplying aid) it cannot be said that the CIA or any other western intel service was ever supporting Taliban, or even "Arabs" as such in Afghanistan.