Shannen, everything happens pretty quickly as the ball is intercepted by Shields, but here is what I’d say. Shields is in front of Knox and touches the ball — not necessarily intercepts it yet at that instant (as in possesses and controls the ball), but at least gets his hands on it — before his fellow Packers defensive back, Collins, runs through the Bears’ receiver, Knox. Just as in the case of a pass that gets tipped by a defensive lineman after the quarterback releases it (even if the tip is barely noticeable and doesn’t affect the trajectory of the pass), there cannot be pass interference on a ball that is touched by another defensive player before the defensive player in question (here, Collins) hits the receiver. Once the ball is touched by a defensive player, it’s considered to be a situation similar to a fumble before it’s been recovered — i.e., the refs permit all kinds of pushing, grabbing and holding that would not be allowed if someone had possession of the ball. (Note, the rule is different if the defensive player who first touches the ball is the same defensive player who has hit the receiver — that would be pass-interference.)
Having watched the play a few times now, I think if Shields had clearly intercepted the ball a fraction or two of a second earlier, there could have been a case for calling clipping (i.e., blocking a player blindside) against Collins. But because it all happened so fast — i.e., it couldn’t have been clear to Collins that Shields had intercepted the ball and therefore that Knox was a potential tackler at that point — I don’t think the refs could have called that.
I’m glad you didn’t ask where my Jets were in the first half last night. That is a question I could not have answered.