In To Kill a Mockingbird why does Helen Robinson fall down when Atticus drives up to her house?
This site should help you with that.
http://www.sparknotes.com/lit/mocking/
She realizes that Tom has been killed.
In the novel To Kill a Mockingbird, Helen Robinson falls down when Atticus drives up to her house because she receives the devastating news of her husband Tom Robinson's death. Atticus Finch, who is a lawyer and a close friend of the Robinson family, had to inform Helen personally and in person.
To find this answer, one can read the relevant chapter or sections of the book where the event occurs. In this case, you can refer to Chapter 25, where Atticus visits the Robinson family to break the tragic news. Reading the book will provide a more detailed understanding of the circumstances leading up to Helen Robinson's emotional reaction.