In Chapter 15 of To Kill a Mockingbird, what's really happening when Atticus moves back toward the porch and the crowd draws in?
When the men come to the Finch house one evening after the family has had supper, they want Atticus to get a change of venue for the Tom Robinson trial, and they express dismay when he rejects the idea.
After Atticus steps outside as requested by Heck Tate, the sheriff informs Atticus that Tom is being moved to the county jail the following day, adding,
"I don't look for any trouble, but I can't guarantee there won't be any..."
Atticus tries to allay the sheriff's fears by reminding him that they are in Maycomb. Nevertheless, Sheriff Tate is uneasy. Still, Atticus makes light of the situation, saying he does not think that they will begrudge him a client. However, Link Deas interjects that he is worried about the Old Sarum bunch, and he asks if Atticus can obtain a change of venue, adding that Atticus has much to lose in the case against Tom. Of course, Link is suggesting that Atticus not defend Tom.
"Link that boy might go to the chair, but he's not going till the truth's told.... And you know what the truth is."
When the men murmur ominously, Atticus steps back toward the bottom step of his house and they move closer.
These actions indicate that Atticus understands that the men feel strongly about the trial, and he wants to pull back and keep the men from doing something that they may regret. He has hoped that by standing on the steps the men will take his hint that he wants to end any discussion and to go back inside.
After this encounter with the men of the town, Atticus realizes that he must keep a lookout at Tom's jail cell that night. Furthermore, this minor confrontation foreshadows the mob scene later at the jailhouse.
What happens in this chapter is that a group of men come to Atticus' house to remonstrate with him and to try and persuade him that there is going to be trouble. Link in particular almost threatens Atticus, suggesting that he has "everything to lose" by taking the case and trying to protect Tom. Even though the group of men say they fear the problems that other men in the town might cause, it is clear that they become threatened, especially when Atticus seems to stand up to them and challenges them. Note how they respond when Atticus asks his "dangerous question," which is "Do you really think so?" The men, obviously detecting that Atticus is not going to back down, begin to make rather disturbing noises:
There was a murmur among the group of men, made more ominous when Atticus moved back to the bottom front step and the men drew nearer to him.
Atticus is clearly beginning to become slightly worried, and so feels the need to draw back as the men press in on him, which only makes the sound that Scout can hear from them more worrying. Atticus is threfore drawing back at this point out of concern, as it is clear that the crowd is upset with his response and the fact that he will not change his stance towards the case at all.