What makes Miss Maudie angry?

It takes an awful lot to get Miss Maudie angry. She's normally such a calm, unflappable woman, unable to get mad even when her house is burning down before her very eyes. Yet in chapter 24 of To Kill a Mockingbird, she does indeed display anger, albeit with an icy chill rather than boiling rage.
Miss Maudie is over at Atticus's place, attending a meeting of Aunt Alexandra's missionary circle. The ladies proceed to show their total hypocrisy when they express concern for the suffering of a remote African tribe while making racist, derogatory comments about African Americans in Maycomb. To this end, one of the ladies present, Mrs. Merriweather, makes a snide reference to certain people in town stirring up black people. She doesn't openly say so, but it's obvious that she's referring to Atticus.
Miss Maudie is none too impressed with this remark, not least because Mrs. Merriweather is sitting there stuffing her face with food bought for the occasion by Atticus himself. Her comments are not just racist; they show a total disrespect for Atticus's gracious hospitality. So Miss Maudie gives it to the hapless Mrs. Merriweather with both barrels:

His food doesn't stick going down, does it?