Why do the men go in on Atticus in To Kill a Mockingbird by Harper Lee?
Your question is a little vague and I'm not sure exactly what "go in on" means; however, there are two instances when a group of men gather around Atticus and both of them occur in chapter 15 of Harper Lee's To Kill a Mockingbird.
By this chapter, Tom Robinson's trial for rape is imminent and the tension is mounting, even among friends. Dill has just arrived unexpectedly in Maycomb and Aunt Alexandra has also made her appearance; things are calm for a bit. Scout introduces the change this way:
We had a week of peace together. After that, little, it seemed. A nightmare was upon us. It began one evening after supper.
A group of men have gathered quietly on the Finches' front lawn, and the children, particularly Jem, are upset at the sight. They know that
[i]n Maycomb, grown men stood outside in the front yard for only two reasons: death and politics.
Unlike his children, Atticus is not upset. (We should not be surprised at that fact, since Atticus has proven himself to be essentially unflappable.) When he goes outside to greet the men, he learns that they are concerned about the potential trouble when Tom Robinson is moved to the Maycomb jail tomorrow.
Sheriff Heck Tate is among the men, and he is especially concerned that trouble might come from a group of men from Old Sarum, particularly if they get drunk and decide to create a disturbance of some kind. Again, Atticus is not concerned about any of this, refusing to believe that anyone would try to do harm to a man sitting in jail--even a black man.
As the men continue talking, we learn that some of them are disgruntled because Atticus has taken this case. Everyone seems to know that there is no chance at all that a black man will win a trial in which he is accused of rape by a white woman, and there is an air of discontent among the men because Atticus has brought this potential trouble on them.
Link Deas suggests that Atticus has nothing to gain by taking this case; and Atticus has a clear response for Deas and the others:
[T]hat boy [Tom[ might go to the chair, but he’s not going till the truth’s told.” Atticus’s voice was even. “And you know what the truth is.”
This comment elicits an immediate response from the friends and neighbors gathered on the lawn:
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.
This is probably the moment to which your question refers, and it is unclear exactly what the men intended. What is clear is that they are upset and they blame Atticus for how they're feeling. While there is a hint of a threat indicated by their closing on on Atticus, it is probably more about frustration or even hoping to change Atticus's mind more than it is about any kind of intimidation or threat. This meeting is about both death and politics, but it is certainly not about Atticus's death.
We'll never really know what the men would have done after closing in on Atticus because Jem, who is watching from inside and is a little afraid of what he sees, hollers at his father that he is wanted on the phone. This makes all the men laugh just a bit, and that laughter eases the tension.
The other incident occurs at the jailhouse. While the Old Sarum bunch does circle around Atticus, they do not get any closer before the children interrupt the gathering. Of course there might have been an altercation of some sort if there had been no interruption, but again we'll never know for sure.