Atticus lectured the children about trying to live up to their name why did this upset scout and jem

Atticus makes an attempt to lecture them about trying to live up their name after being pressured by Aunt Alexandra, who's known to judge families based on how long they live in one place for generations.

This upsets Jem and Scout because it is a strange side of their father that she has never seen before, that's why Atticus returns to his original principle.

(To Kill a Mockingbird - Harper Lee)