That's animation lock happening to the mandies first off. Second I can guarantee you that if he had been actively engaged with them before the final nuke, he would have likely taken a hit before the mob died. Cause the game would still be processing the attack before the death. And this game does the exact same thing. A mob can be dropped instantly if its hit hard enough and can get a swing off if its in the middle of attacking while dying. The exception to this is their telegraphed moves which can still display the animation (animation lock again) but they're dead so it didn't actually go off

And as for mobs linking, it's obviously built into the system that if you are overpowered for a zone it causes weaker mobs to link to your target. It does the same thing if you're in a party. Why is up to anyone's guess, perhaps the devs thought it would up the nonexistent danger factor