Can't stand the supposed pundits, sportscasters and radio talk show call in fans. Everyone who has ever played baseball knows that it is the force of the ball (it's momentum that starts the natural process of the glove closing during the catch. Short of Ricky Henderson's snatch catches (flash showmanship only) nobody closes the glove on purpose unless the ball is entering the mitt and most of the closure activity is caused by the ball.
Yes the Fan did interfere with Mookie and push on his glove which closed the pocket. That said Mookie's glove may have been in home run territory. Worst case for the Red Sox it should have been a live ball knocked back into play. Best case Fan interference. I think they got it right!