This is a good question. At this point in the novel, we know two important things about Ma - she is the family's "rock" and she is starting to crumble.
Early in the novel, Ma recognizes the fact that she is the strength of the family. Here, however, we see that her strength is being challenged by deep worry. She has already threatened her husband with a tire iron in a move to keep the family together on the road. Now she threatens a uniformed police officer.
She also admits that her reason for turning away the Jehovites is that she is afraid of breaking down. Her strength, which the family counts on, is clearly at risk. This risk adds significant tension to the story and depicts Ma as poised to lose control of herself. Her emotional restraint is nearly gone.