DoubleBogey is probably correct to suggest that as the Nr reduce ,due to a high blade pitch angle,the hydraulic output of the pump(s)?) will reduce and be unable to overpower the loading on the jacks,leading to jackstall and then loss of all hydraulic pressure as the Nr reduce further.
It is possible to recover RRPM also by rolling into a descending turn and applying `g`; also helps in situations where ice may be building ,not shedding,on the blades,and the drag rise following an engine failure may well drag RRPM rapidly down.