Here's why:
* The attack: Bob Ewell, seeking revenge for Atticus defending Tom Robinson, attacks Jem and Scout on their way home from a Halloween pageant.
* Atticus's appearance: While the children are being attacked, Atticus arrives, seemingly out of nowhere, and quickly disarms Ewell.
* The surprising skill: Atticus, known for his pacifism and legal profession, demonstrates incredible physical prowess and fighting ability. He effectively subdues Ewell despite being older and seemingly unathletic.
This moment shows the children that their father is not just a lawyer and a gentle soul, but also a capable and protective individual, even in a physical confrontation. It shatters their prior perception of him and leaves them awestruck.