why do mama and Mr.Morrison get out their firearms in this chapter?

Are you referring to chapter 3? Mr. Avery appears at their door and announces that "They's ridin' t'night." Mama sends the children straight to bed. Cassie sneaks out and watches what is happening. She later sneaks back into her room and pretends to be asleep as Big Ma pulls a shotgun out from under the bed and sits at the window. Mr. Morrison stands guard as well.