This is an excellent question concerning one of the novel's most dramatic events. Atticus has already been warned that there might be trouble at the jail where Tom Robinson is being held. Link Deas isn't concerned about local Maycomb residents:

"It's that Old Sarum bunch I'm worried about..."  (Chapter 15)

Deas is right: As Jem, Scout and Dill cut across the courthouse square, they see a "line" of cars approaching the jail. It turns out to be the Old Sarum boys, led by Walter Cunningham Sr. They have sent Sheriff Heck Tate on a "snipe hunt"--in this case, a bogus report for a non-existent crime--and plan to take Tom from his cell and lynch him. They may have suspected that Atticus would be waiting for them. With Tate out of the picture, the jail could have been left unattended, and it would take only minimal manpower to remove Tom. But the Old Sarum bunch (with their Ku Klux Klan overtones) come in force--"four dusty cars" filled with men. Why would they bring four cars when one or two is sufficient unless they knew that someone--Atticus--would still be there? Perhaps Cunningham and his men realize that Atticus will be more difficult to convince to step aside and give up Tom than Sheriff Tate. In any case, as many as a dozen men converge upon Atticus, who nonchalantly converses with them and demands that they speak softly so as not to "wake him (Tom) up." A lone man defending his client, Atticus has the respect of the other men and is in control of the situation--though violence appears imminent--until the unexpected arrival of the children changes the course of the evening.