From the MLBUM: "If, after a player has fielded a batted ball but before he is able to throw the ball, a runner hinders or impedes such fielder, the runner shall be called out for interference."
Seems to me that's what happened here. F6 fielded the ball, but before he was able to throw it (which he undoubtedly would), R2 ran into him. If R2 had knocked F6 to the ground, allowing R3 to easily score, how would you not consider that as interference?
Train wrecks happen when a thrown ball causes a fielder to get into the runner's path, and the fielder, runner, and ball all arrive at the same place simultaneously. They also happen when the catcher and batter-runner make contact on a ball in front of the plate. They do NOT happen when a fielder has long had possession of the ball and he's running to make a play. The fielder is under no obligation to tag a runner approaching him, so just because that didn't happen doesn't excuse R2 for running into him.