This question is never answered clearly through Scout's narration in To Kill a Mockingbird. It can be assumed that Aunt Alexandra badgered her way into joining the family, and Atticus seems to have decided not to try and oppose her authoritative manner. Presumably, Atticus knew he would be busy with preparations for the Tom Robinson trial and that he would have less time to spend with his children than usual. He would also be away in Montgomery for legislative responsibilities as well. Alexandra tells Jem and Scout that she and her brother had "agreed" for her to come, and that Scout needed "some feminine influence." Alexandra also seems to be perfectly happy to be away from her husband, Jimmy, who she left alone at Finch's Landing. As Atticus explained in a fumbling way,
"We felt as if it were time you children needed--well, it's like this, Scout... Your aunt's doing me a favor as well as you all. I can't stay here all day with you, and the summer's going to be a hot one."
"Not understanding a word he said," Scout decided that
Aunt Alexandra's appearance on the scene was not so much Atticus's doing as hers. Aunty had a way of declaring What Is Best For The Family, and I suppose her coming to live with us was in that category.
Scout didn't appreciate having another adult to boss her around, and she battled with her aunt concerning her obsession with heredity--"Gentle Breeding"--and making Scout a lady. Jem stayed out of her way and warned Scout not to antagonize Alexandra. It got so bad that Scout considered running away from home, but she abandoned her plans when Dill decided to run away from home himself and showed up under her bed one night.