Siloe sid, think back to the vector diagram of a blade in auto rotation. If the Nr gets below the minimum 80% in this case you are not going to get it back. High rate of descent gives a very large angle of attack on the blade, the resultant drag will slow the blades very quickly. Disengaged blades on the ground have no rate of descent flow, and a very small angle of attack, so take a while to slow down. I am talking simply about p of f and not speculating on what happened.