I tried to search for this, and could not find anything. I am sure someone has posted about it. Either way...
When an NPC runs while low health, they seem to have a speed boost. They move faster than you can move, which makes it very hard for melee to finish them off.
Also, the "linked" pull seems to have a larger radius. It seems like they are more likely to pull multiple instead of just the one.
It has been a long time since I've played retail so maybe I am just not remembering it correctly.
It's very annoying indeed. I remember mobs having a lower than normal runspeed when fleeing on retail. Anyways, it's a good reason to use Crippling Poison or Hamstring or whatever you have and to keep your ranged weapon skill high, so you can snipe mobs as soon as they're out of melee range.