Lands of MysteryThis section concerns content exclusive to the Warcraft RPG, and thus is not necessarily canon.
According to Lands of Mystery, after Brann Bronzebeard visited the dwarves of the digsite he knew he needed to act before his friends were put in greater danger. While he visited Thunder Bluff he suggested to Cairne Bloodhoof that he send a group of his own people to work with the dwarves in exchange for making sure that the land was properly cared for. Cairne agreed to bring up his offer to his council of elders for discussion. Apparently it worked, as Baine Bloodhoof asked a group of young tauren to act as emissaries to the dwarven excavation of Bael’dun to convince the dwarves to cease their careless digging. Given enough effort, the tauren apparently convinced the dwarves to allow a shaman to oversee their site to make sure that the land is properly cared for during the excavation, and that the dwarves return to the land as much as they take.
CataclysmThis section concerns content exclusive to Cataclysm.
Cataclysm however paints a different picture of the events leading up to now. In Cataclysm, it is revealed that the dwarves refused to heed their taurens warnings and their reckless digging infuriated the spirits of the land who killed them. Ahmo Thunderhorn now sends tauren to calm the Earth spirits in the ruined dig site with a magic drum.