I think that the reaction that Changez gives to the falling of the towers helps to create a turning point because it represents the point at which Changez begins to externalize a dislike for America. Prior to this point, Changez had internalized the disconnect between he and the nation that had been home to his prosperity. He had done everything and represented everything about "the American Dream" and, more importantly, "the immigrant dream." It becomes the turning point because his reaction to what he sees on television starts a strong break between he and America. It is a break that starts to demarcate where that line of division exists between both. It is a turning point because from this point on, there is an eventual dissolution between Changez and America. It had never been as pronounced as the moment when he sees the Towers fall. It is a turning point because one can identify this moment as the essence of when the arc of Changez's characterization makes a distinct turn from what it was to what it will become. I think that this becomes why the scene can be understood as a potential turning point in the narrative.