Weird Dropped Third Strike

[shimes]

How about this one: Two outs, two strikes. The batter swings at strike three, the ball goes into the catcher's glove and then squirts out directly in front of him--the catcher doesn't secure the ball for the third strike to be an out. The squirts out, and while still in the air, the batter's natural backswing knocks the ball over toward the dugout. The batter starts running to first, and I kill the play.

I called the batter out for interference, but darned if I know why. I couldn't think of any specific rule that covers an odd-ball play like this, but instinctually, it just seemed like the only possible call. I explained to the coach that the batter had the right to run to first, but at the moment the ball contacted the bat and knocked it thirty feet from the catcher, he interfered with the catcher's ability to field the the third strike.

I guess this one might cover the play, but I don't know. Any ideas?

7.09 It is interference by a batter or a runner when-

(a) After a third strike he hinders the catcher in his attempt to field the ball;