It is part of the problem with having an aerodynamic item (ie a helicopter) underneath another helicopter - it wants to fly and is badly affected by the swirl of the downwash. Add in some crosswind and it can get quite sporty.
However, using more lines attached to the 'load' would have helped the ground team keep it all in order.
I watched a Lynx, underslung from a Chinook start to swing and then suddenly fly back and up with the tail boom of the Lynx coming very close to the rear rotor of the Chinook. South Armagh circa 1985.