The grave could be a place for the Crime Familes to dispose of bodies, as two Sindacco Family thugs (depicted in "The Introduction") are seen in the desert, digging a grave to dispose of a body, although it's one person, not six - possibly indicating that more people were buried there during the events of GTA San Andreas.
The person who created the graveyard here could be Mary-Beth Maybell, from the radio station K-Rose. She says that she has six husbands, all of whom she has killed and says that they are now "six feet under". There are six body bags in the grave. She also says that she broadcasts somewhere in Bone County, which is a desert, and the same location of the mass grave. She also says she drives a truck, which may explain the nearby parked Bobcat.
Rockstar unofficially announced the answer to the question of the body bag myth. "We used these body bags as an easter egg, or as you wish to call it, a reference, to real-life mafias from Las Vegas dumping dead bodies in the desert."