I've no great experience on the Huey, although my first flight in one was carrying a netted USL.
However, collective/ vertical bounce is a known phenomena on another aircraft I've operated; for example:
On the Blackhawk it can occur where the pilot himself is vertically "bounced" by the aircraft when lifting loads. The initial bouncing he suffers causes him to inadvertently induce small, but in-phase, collective inputs. This may result in the rapid amplification of naturally occurring vertical vibrations when picking up external loads. It can rapidly become a runaway situation causing major or even terminal airframe damage. The cure is to briefly let go of the collective and to screw on some more friction, which breaks the cycle.
I recall Sikorsky sending out a bulletin on this in the mid 1990s. Around that time our unit lost a big firebucket to the bottom of a deep lake, after a trainee pilot froze on the controls whilst experiencing the phenomenon during load pickup (6,000 lbs of water). The instructor found it necessary to jettison the load to recover the aircraft from a rapidly deteriorating situation.