Perhaps the rebound is explained by the angle from the hoist - if the aircraft climbs to deploy the hi-line then the angle is steep and the hook has a long way to go vertically to reach the rotor.
If, on the other hand, the aircraft is moved laterally as the hi-line is deployed, the vertical distance is much lower - ISTR the report mentions the winch wire being snatched from the winchop's hand - that would correlate with the hi-line snagging and then parting.
Or it could well be a problem with the hi-line itself or a combination of the two issues.