Why did Walter Cunningham and company come to the Maycomb jail?

In chapter 15, Walter Cunningham and the Old Sarum bunch travel to the Maycomb jailhouse in order to lynch Tom Robinson before the trial. When they arrive at the jailhouse, they find Atticus sitting on a chair, calmly reading a newspaper outside of Tom Robinson's cell. Jem, Scout, and Dill watch from their hiding spot across the street as the Old Sarum bunch surround Atticus. Walter Cunningham demands that Atticus move out of the way and allow them to get to Tom Robinson, but Atticus refuses to leave the jailhouse. Walter also attempts to intimidate Atticus by telling him that they called Sheriff Tate off on a snipe hunt so that he wouldn't be around. While the men are talking, Scout gets curious and ends up running into the middle of the group. Everyone is initially shocked at Scout's presence, and Jem and Dill also follow her out of their hiding spot. Fortunately, Walter Cunningham ends up sympathizing with Atticus's difficult situation and instructs the mob to leave without harming anyone.