Aunt Alexandra feels guilty because she had believed Bob Ewell might eventually try something like this. In Chapter 29, she says:
"Atticus, I had a feeling about this tonight—I—this is my fault,” she began. “I should have—”
When Bob Ewell was harassing Helen on her way to work, Aunt Alexandra was worried that he might take out his frustrations on anyone involved in Tom Robinson's trial. She doesn't understand why Bob Ewell would continue to hold a grudge considering that his side won the case. Atticus thinks Bob's grudge is based on the fact that most people didn't believe Bob's and Mayella's lies. Atticus also thinks that Bob realizes he was made to look like a fool in court and for these reasons, Bob continues to hold a grudge. However, Atticus did not think Bob would stoop so low as to attack his children. Aunt Alexandra, on the other hand, said:
“I don’t like it, Atticus, I don’t like it at all,” was Aunt Alexandra’s assessment of these events. “That man seems to have a permanent running grudge against everybody connected with that case. I know how that kind are about paying off grudges . . . " (Chapter 27)
Aunt Alexandra thought that Maycomb had put too much pressure on Atticus to defend Tom. As defender of a black man in a relatively racist town, Alexandra later thought the town put Atticus in a dangerous position. For these reasons she had an intuition that something dangerous might happen. She didn't pursue this intuition other than voicing concern; this is why she has a moment of guilt after the children are attacked. But Mr. Tate reassures her that acting on every intuition would create chaos; therefore, she shouldn't worry about predicting what could or could not happen.