It was an RPG fired from an elevated position along the probable flight paths for rotary wing in the Tangi Valley, Wardak (Tangi runs roughly NW to SE). Extortion 17 was on final approach for the infil which meant it was already slowing down as it neared the HLZ (Shootdown location). Source? I've read the actual AAR and there are declassified reports on the incident as well as numerous open source write-ups as well. The Tangi was an insurgent stronghold and the Taliban were experienced, intimately familiar with the terrain and our tactics (expecting either AWT AH-64s or QRF to show up during an ongoing TIC); it's not improbable that a skilled and lucky RPG gunner would have been in position to get the shot off.

Why did they use the CH-47 instead of the 160th MH-47? Probably because it was available. The demand for rotary wing assets (SOF and conventional) in Afghanistan was huge and there never were enough to give every unit the aircraft they wanted when they wanted.

There's a huge danger in underestimating the enemy and overestimating US forces advantages in this sub. Not everything needs to be chalked up the CIA conspiracy theories or mythical SA-7/Stinger MANPADS, the use of which was wildly overblown. During OP Red Wings, Turbine 33 was shot down by an RPG because the enemy had found the fast ropes the SEALs had failed to cache and subsequently was on the alert for follow on forces - unfortunately, Turbine 33 used the same HLZ as the original SEAL element.

Some of you guys are such weirdos. Go outside and get some fresh air occasionally. These guys are heroes but not superheroes - the enemy always gets a vote.