In chapter 16, how is the relationship between Atticus and Aunt Alexandra changing?

In Chapter Sixteen, Atticus stands up to Aunt Alexandra and her racist views. Hitherto, he's put up with her verbal attacks and generally shrugged them off. In this chapter, he stands up for Calpurnia and her race and allows himself to show his irritation.

She waited until Calpurnia was in the kitchen, then she said, “Don’t talk like that in front of them.”
“Talk like what in front of whom?” he asked.
“Like that in front of Calpurnia. You said Braxton Underwood despises Negroes right in front of her.”
“Well, I’m sure Cal knows it. Everybody in Maycomb knows it.”
I was beginning to notice a subtle change in my father these days, that came out when he talked with Aunt Alexandra. It was a quiet digging in, never outright irritation. There was a faint starchiness in his voice when he said, “Anything fit to say at the table’s fit to say in front of Calpurnia. She knows what she means to this family.”
“I don’t think it’s a good habit, Atticus. It encourages them. You know how they talk among themselves. Every thing that happens in this town’s out to the Quarters before sundown.”