[Tru_in_Blu]

Well, since it was a foul pop up, the runner from third wouldn't (shouldn't) have "been in motion". If anything, she probably retreated to the base in order to tag up.

I agree that on a routine pop up to an infielder, a runner is unlikely to try and tag up.

However, once F3 was wiped out (accidentally, inadvertently, unintentionally), now the runner made the decision to risk advancing. So once the fielder went down, as a result of a retired (batter) runner INT, we have nothing?

So in the case play I referenced, the ruling is:
If, in the umpire's judgment, B2 hindered F3's play on R1, R1 is declared out.

I can't see the defensive coach being quiet about this if the run is allowed to score given that his fielder was knocked over.

And I don't know how you return the runner to third base if INT wasn't called.

So how do you reconcile this?