[Bishopcolle]

Don't remember the pro game, but a bunt to first trying to score the runner at 3rd is fielded by F3....he quickly heads to the plate and stops R1 who heads back to 3rd base, only to find R2 standing there....He does not step on the bag, but he gets very close and the BU calls him out on the tag...

Question: If R1 grabs the bag, and R2 stays on it too....who is ruled out on a double tag? Does R1 have to vacate? or R2?
What is the mechanic?