Why does jack betray Auggie by talking behind his back?

Jack betrays Auggie by talking behind his back due to peer pressure and a desire to fit in with his classmates. Initially, Jack connects with Auggie but later feels torn between his friendship with Auggie and the need for acceptance from others. His comments stem from insecurity and the fear of being ostracized for associating with someone who is different. Ultimately, this betrayal highlights the challenges of navigating friendships during adolescence.