It's an RNG quest in Grizzly Hills. You have to use Gavrock's Runebreaker on Runed Giants to free them. It has a chance to work, or a chance to "weaken" them, normally resulting in you grinding the weakened giants and looking for another one to try the runebreaker on.
If you break combat with the weakened mobs, through Shadowmeld, Feign Death, Vanish, whatever, they will reset then turn back into Elite Runed Giants. You can then use the runebreaker on the same giant until it frees them.
It's just a basic exploit, but certainly not intended mechanics methinks.