Originally Posted by Otou

The heal actually does prioritize injured players, it's just not triggered by other players taking damage / low health. Their chance to explode on each melee hit, is increased by their master's health being low. I don't know the exact chance, but the lower your health, the higher their chance to explode. From there it heals the 6 most injured players in range, in addition to the master if he's close.

It would be more effective if they were triggered by the players in range, rather than their master. On large bosses, the Bloodworms are out of range of a tank anyway. So exploding based on the tank's needs isn't very effective for anyone.