Graveyard zerging is the act of overwhelming an enemy (usually a world boss) by having players immediately run back to their corpse and resurrect during a battle, rather than use any sort of strategy or skill in the battle. The point is to keep a neverending stream of players attacking a target until it's dead.
This was once a major problem with the world bosses, but Blizzard's solution was to give players a 15-minute debuff when they die which freezes/sleeps the player if they get too close to the World Boss. The Four Dragons and Azuregos had this feature. Lord Kazzak becomes nearly immortal after 3 minutes of battle, and is healed each time he kills a player.
Unlike vanilla world bosses, graveyard zerging is a viable "tactic" for Mists of Pandaria world bosses — namely, the Sha of Anger and Galleon — as they are intended to be casual-friendly. Oondasta's spawn position is roughly 90 seconds away from the graveyard (even though it's only a few hundred yards to the south), so it is possible to zerg it as well.