The group of men from Old Sarum (mostly Cunningham family members, apparently) showed up at the jail with the intent of lynching Tom Robinson. Atticus had been warned by some townspeople earlier that Tom's stay in the local jail may not be safe, and there is also mention of the Ku Klux Klan afterward. It is safe to assume that the men were planning to take Tom from the jail by force while Sheriff Heck Tate was out "on a snipe hunt"--investigating a bogus call that sent him "deep in the woods" deliberately meant to keep him away from the jail. As in many scenes in To Kill a Mockingbird, the darkness of the night is meant to hide the activities of its characters (Boo prowling around at night, Jem paying night visits to the Radley house). The lynch mob probably thought it would be safer to take Tom and hang him in the shadows of the night in order to better protect their own identities .