Why does Aunt Alexandra get upset with Atticus for talking about Mr. Underwood in front of Calpurnia? What does this show about Aunt Alexandra's character?

Aunt Alexandra doesn't want Calpurnia stirring other Negro's up with news she hears at the Finch house. This shows Aunt Alexandra's clear misreading of Calpurnia as well as her ignorance. Aunt Alexandra is a product of the distant past.