It is the understanding of the citizenry of Maycomb that Atticus Finch violates the unwritten social laws of their society. For, in defending Tom Robinson, Atticus takes the word of a black man over that of a white man, notwithstanding the fact that Tom is innocent. Indeed, Atticus's intentions of actually preparing a legal defense for Tom rattles the very pillars of Maycomb's social stratum.
When the townspeople learn of Atticus's plan to fully defend Tom, several of them resort to calling him a "n****-lover," including his own nephew Francis and neighbor Mrs. Dubose. Dill's Aunt Rachel says, "If a man like Atticus Finch wants to butt his head against a stone wall, it's his head." And, at the missionary tea that Aunt Alexandra holds, Mrs. Merriweather also expresses her disapproval, "I tell you there are some good but misguided people in this town." Clearly, then, the townspeople of Maycomb feel that Atticus violates their cultural rules. In a sense, then, they perceive his actions as an act of betrayal.
Scout, early on in the novel, calls Maycomb a "tired old town" and her description of the town adequately describes the minds and moral beliefs of many of the citizens who live in Maycomb. As Maycomb fits Aunt Alexandra "like a glove," it in contrast, barely fits Atticus at all. The majority of the townspeople in Maycomb are averse to change and radical ideas of moral thinking. Miss Maudie, Mr. Dolphus Raymond, and even Mr. Link Deas are all examples of characters who must suffer scorn from some aspect of the town for the ways that they chose to live their lives. Atticus choosing to properly defend a black man (instead of throwing the case) and thereby implying that the words of white citizens are lies is breaking their town's old societal code of racism.
Atticus is mentally and physically shunned by the townsfolk and his children are subsequently bullied by the other children for his decision to do the morally "right" thing. Aunt Alexandra feels he is a disgrace to his family, and Mrs. Dubose even chastises the Jem and Scout about the nature of their father's "radical" decisions and actions. In short, the tired old minds of Maycomb that are stagnant and resistant to change and liberal thinking about the human race, feel that Atticus is deserting them for the dishonorable life of a "n***** lover."