I have heard of at least four aircraft in the last six months having hard landings some severe, luckily with no fatalities.
In each event the pilot stated that they did not stiff arm the collective. I am sure they probably did not move their arm in relation to their body. If their body was moving without moving their arm they were inducing biomechanical feedback into the system.
In one occasion the bounce was so severe that the pilots feet came off the pedals. This situation of biomechanical feedback is also mentioned in the NTSB report DCA16FA199 of the July 06, 2016 accident of the Bell 525 test aircraft as a major factor.
Possible Mechanical factors The connection where the input rod connects to the servo all the bolts to include those on the servo should turn freely by hand after keying the connecting hardware. ( in the book ) If this hardware is tightened too tight it will cause feedback in the control.
I have seen on many aircraft that the head of the Pilots collective stick additional switches / buttons have been added.
This adds to the weight at the end of a long arm, making it hard to keep the collective from dropping in flight. Making it hard or impossible to correct with friction, preset or Pilot applied.
One mechanical item that can effect this is not enough built in or ground adjustable friction. ( TM 55-1520-210-23-2 pages included ) & ( TM 55-1520-242-MTF page included )