OK: here goes:
Lower the collective and the amount of pitch deselected in the rotor head causes a reduction in lift due to that reduction in AoA.
The lift reduction is dissimilar (unequal) around the rotor plane due to the required AoA's on the retreating and advancing side.
More lift is lost on one side compared to the other side due to this reduction in AoA - as a proportion of lift generated in those areas.
Due to phase lag the net result in dissimilar lift in the rotor head rotates "x" degrees and causes the helo to pitch up or down dependent on the direction of rotation of the rotor head.
This phenomena is not divergent and the overall lift almost instantly equals out around the disc and it returns to a steady state.
The mention of stabilisers is a secondary effect on those helos that have them fitted.
See posts #9 and #24 in the previous posters link.
Last edited by Thomas coupling; 13th November 2013 at 16:43.