Jack, being loyal and protective of Auggie, is angry at Julian's relentless teasing and unkindness towards Auggie. The punch is a physical manifestation of Jack's anger and frustration, and it shows how deeply he cares for Auggie.
It's important to note that this is a complex situation with multiple layers. While Jack's action is motivated by good intentions, it is still an act of violence and has consequences. The book explores the complexities of friendship, loyalty, bullying, and the importance of standing up for what is right.