Just thinking out loud

[greymule]

a 20+ year HS/college veteran

If your partner does NCAA, this could explain his apparently erroneous LBR call in the HS game.

NCAA (12-21-b): When a runner is legally off her base after a pitch or as a result of a batter completing her turn at bat, and she is in motion as the pitcher takes possession of the ball within the pitcher's circle, the runner may continue moving forward in the direction she is going without stopping, or she may immediately and directly go back in the other direction.

NCAA doesn't allow the blanket "one stop" that Fed and ASA do. In NCAA, if a runner remains in motion past what would be an "immediate" time, she has to continue without stopping. ("Immediate" is not really defined, but I give the benefit of the doubt to the runner—she would have to remain in motion for a significant time to be committed to continuing without stopping. If she was in motion and then reversed after 3 fast steps, I'd consider that "immediate.")

In the play in the OP, if the runner was in motion back to 3B when F1 got the ball in the circle, and the runner remained in motion—either for several steps or walking slowly for a significant time—and then stopped, the ump might have had NCAA on his mind and called her out.