Two Runners On Same Base

[fredhjr]

Dodgers vs Cardinals on JUN 01, 2017--R1 on 3rd, R2 on 2nd. B1 hits ball (chopper or slow roller) to F5. R1 starts running toward home, F5 throws to F2 and R1 is now in a rundown. Back and forth between 3rd and home a few times. Finally, R1 dives back to 3rd base. During the rundown, R2 has run to 3rd base. Now, both players occupy the base. Although I did not see a tag, there must have been one because R1 was the player called out (if there had not been a tag, R2 should have been called out if I understand the rule correctly). I did not see an explanation from the umpire, although there was a small discussion between him and the third base coach. Just wondering if anyone else saw the play.