Was the interference called on the play back to first or the play on the runner advancing to 2nd? If he called it because she interfered on a defender being able to make a play on the runner advancing to second I can see it. The batter un-intentionally does knock the ball down preventing it from "possibly" bouncing to the catcher who now had to run down toward first to get the ball. I am not saying it is the correct call, but I could see why he made it. Thanks for posting, it is a rule I had never really seen before.