Interference requires a play to be interfered with.
But, the knock down hindered the fielder's ability to make a play on R1 when R1 advanced or even if R1 did not advance. *
Some interference instances have a delayed effect, not necessarily immediate. It looks like this is that type of case and so interference applies.
The call should have been at the moment of collision, then judge whether the possible double play fits.
Hard to call, hard to explain, but "big bucks".
* Think pickoff or throw home to prevent the score.