Crab,
The aerodynamics of the 44 helped the sling pilot to it's target being a VERY stable load. For the start it looked good, but the swinging of the load was induced by the pilot stirring the pot and not being in control of his load.
The oldest excuse in the book for this, is to blame it on the
wind or downwash and I see it all the time with pilots not being current in longline work or with new pilots.
The ground crew could have helped out a bit with good hand signals though...
Your aerodynamic instability is true if we talk about light plane recovery, boats, parabolic antennas, survival shacks, drill floors, flat packed containers, packs of corrugated roofing sheets etc, not a 5-600 kg (lawn) dart
All external loads have it's own Vne which will ruin your day if you go past or at least scare you, and In your Lynx story the pilot of the Chinook flew past the Lynx's Vne as cargo
The video of the CH53 that dropps the CH47 that got out of control is a perfect example.