There are so many possibilities that have not even been considered here yet - some more improbable than others, but not impossible.
How about something (maybe a set of binos, or the RFM or something) in the cockpit got stuck under the collective at precisely the most inopportune moment, so it wouldnt go down when needed. That could very quickly cause the loss of the Nr and become irrecoverable almost immediately when trying to autorotate. (If we think the engines were off for whatever reason).