The weak link in a hi-line is designed to do exactly that - break - so that any entanglement or snagging doesn't put an undue load on the hoist cable and snap the wire or damage the housing/drum or even pull the hook off the wire.
The fact that this one recoiled towards the rotor isn't unusual, we had one that did go through the rotor after the hook itself snagged a boat rail and then came free under tension.
This incident would be considered a Normal Operating Hazard for military SAR.
Normal operating hazard YES. That much hook rebound from the tension of a 90lds weak link parting NO. I suspect the link did not function correctly hence the buckle breaking instead which must have happened at more than the rated 90lbs.
After all crab most UK mil and Civi weak links are 150lbs breaking strain and even that does not cause the violent rebound seen in this incident.