Pass Interference?

[ChickenOfNC]

Looking for some general opinions. I've been officiating for about 5 years now, so I'm getting a pretty good feel for most aspects of officiating. But IMO, pass interference is one of the toughest calls we make. I'll try to describe a play I had today (middle school).

Team A runs a reverse, and the guy getting the ball pulls up to pass. He has a WR wide open about 40 yards down field. He heaves it as far as he can, but obviously doesn't have the arm to enable the WR to keep running his route. So the WR is standing there waiting on it. The DB was probably 15 yards in front of the WR and starts stumbling back toward the WR, the whole time looking back at the ball. He gets to the WR about a second before the ball. They both sort of stumble together, feet tangled, both looking up for the ball. They both fall, incomplete pass. I rule no foul, incidental contact. Of course, crowd, coaches, etc. go nuts.

I'm sure one could have the opinion that it was the defender who initiated the contact. But he didn't run through the guy, and they were both playing the ball. Again, just looking for some general opinions/philosophies on PI.