Why does Calpurnia change at the church?

Rather than using proper English, she reverts to "colored talk" soas not to offend the blacks. She explains that she doesn't want to seem uppity to the other blacks by talking better than they do. It's part of "walking around in someone else's skin" as Atticus has taught the children: from the blacks' point of view, Cal's good speech would be "white" and therefore condescending to them. So, she uses their talk so that they will all get along.