Throughout most of the novel, Atticus Finch is portrayed as a patient, kind, intelligent, and maybe even soft spoken man. However, as the the stress of the trial wears on Atticus he begins to change. That change is expedited by the mob scene at the jailhouse and maybe even his sister's negative comments about the trial in general and Atticus's decision to take the case in the first place. It's also possible that Atticus feels a little bitter about working so hard on a case that he knows he is likely to lose.
Atticus is normally a pillar of composure in Scout's opinion, so to watch him get angered and frustrated with Aunt Alexandra shows her a shift in his personality. At times it even seems that Atticus is looking to pick a fight with Aunt Alexandra—not a fist fight, but Atticus knows that she disagrees with Scout drinking coffee, yet lets Scout do it in front of Aunt Alexandra (and gets a warning look from her for it).