So suppose it happens in reverse (this play will be a little bit of stretch, maybe there's a better example). R1 on second, R2 on first. Ball popped up to left. Runner at first goes runner on 2nd takes a few steps off the base waiting to see if the ball is caught. R2 is tripped by the 2nd baseman while rounding second. Ball is caught and R1 returns to second retreating behind the now fallen R1. The obstructed runner is now out for passing a runner and we can't fix it. But if the lead runner had been obstructed and R1 had fallen of her own accord, we can?
And just to make things worse, suppose that F7's momentum carries her into dead ball territory.
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GLASS PIPE