Pitch cone coupling is a design characteristic of the 540 rotor system. It is a result of the rotor system coning under a g-load and the flexure plate exerting a down force on the trailing edge p/c links which causes the rotor to unload itself. If you choose to, you can compensate for it by increasing collective thoughout a high G manuever. It is well known by any one with UH-1C/M or AH-1 time. I really don't think it played a part in this accident because the aircraft looked like it went nose first toward the ground, then leveled almost at impact - also given his airspeed and attitude I really don't think there was much of a load on the rotor. For my money I would say he may have had a single system hydraulic failure (a dual failure would be highly unlikely), or he just ran out of altitude and ideas while performing low altitude fancy flyin'. YMMV.