"Atticus,I have a feeling about this tonight-I- this is my fault," she began. Why this is her fault?.

In this part of the story Scout and Jem have just been
attacked on their way home from the pageant at school.  Aunt Alexandra is, of course,
beside herself.  Neither she nor Atticus accompanied the children to the play and likely
this guilt comes as a result of the maternal side of Aunt Alexandra that tells her she
should have either gone with the children to the play, or made them stay
home.

Don't forget that just a few pages before (in Chapter
27) when Scout is giving her mini-performance for Atticus, Alexandra and Cal, Aunt
Alexandra has a chill and says, "Something just walked over my grave."  This was a
moment of foreshadowing that something bad was going to happen later - and when it does
- no doubt Alexandra thinks she is partly responsible because she did nothing back when
she had the foreboding feeling.

In reality, however,
realize that the events of the evening are not in fact Alexandra's fault.  She likely
could have done nothing to stop them even if she had been with the
children.