Nigel and chopjock - I would say it is all about the bending stresses at the root with high pitch angles (very high considering the low Nr). The amount the blades can cone on an EOL is limited by the Nr decaying rapidly - no Vsquared = no lift = no coning. But in powered flight at low Nr the engine keeps them going at high coning angles - much more bending at the root I think.
Wargee - I think your rotor is ingesting its own vortices, not unlike VRS - hence why it get worse if you raise the lever. A mixture of BVI (Blade Vortex Interaction) and incipient VRS.