When Atticus takes the children for their annual Christmas visit to Finch's Landing, he reveals to his brother, Jack, the true nature of how he came to defend Tom Robinson. Tom did not hire Atticus, nor did Atticus seek Tom out. In fact, he admits that he hoped to never take on such a case--one that might make him enemies in the town and put his family in danger. Instead of the regular public defender handling the case, Atticus has been "asked"--more like assigned--by Judge John Taylor to defend Tom, and Atticus realizes that he won't be able to face his children if he does not accept.
"You know, I'd hoped to get through life without a case of this kind, but John Taylor pointed at me and said, "You're It." (Chapter 9)