In Harper Lee's "To Kill a Mockingbird," why is it unusual that Atticus is defending Tom?
Legally speaking, it's not. Atticus was appointed Tom Robinson's case by Judge Taylor, and he accepted to take on the case according to protocol.
What's unusual is Atticus's determination to defend Tom and prove his innocence. Atticus can see through the Evell's dissimulation and he knows only to well how deeply entrenched in racial prejudice the people of Maycomb really are.
Atticus will have no part of a kangaroo court going through the motions of a trial, and this is the real conflict of the story.
Atticus has been appointed to defend Tom, but his veracity in doing so is what makes his stance unusual: At this time in American History, blacks and whites were racially segregated, and prejudice was very strong on both sides of the color line.
Atticus proves to be the exception to that rule, as his defense of Tom Robinson is done with great professionalism and perseverance. For a white man of the 1930s south to defend an African-American in such a way was unheard of, at the very least.