Why do Big Ma and Mr.Morrison get out their firearms in this chapter?

Mr. Avery appears at their door and announces that "They's ridin' t'night." Mama sends the children straight to bed, but Cassie sneaks around the outside of the house into the boys' room. They hear that the "devilish night men" have been "set off." Cassie worries that the men are coming after them because of what they did to the bus, and Stacey feels as if it were his fault because it was his idea to dig the ditch.

Cassie 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.