In Harper Lee's To Kill a Mockingbird, there are several things that concern Aunt Alexandra about Bob Ewell's behavior.
Angered...by his exposure on the witness stand, Ewell makes threats to Atticus & others involved in the trial...
The first concern may arise when Ewell gets fired from his WPA job—employment provided to people during the Great Depression. He starts receiving welfare checks again and according to Ruth Jones, the welfare lady...
Mr. Ewell openly accused Atticus of getting his job. She was upset enough to walk down to Atticus' office and tell him about it.
Judge Taylor's house is broken into, while the Judge is home alone, reading.
One Sunday night, lost in fruity metaphors and florid diction, Judge Taylor's attention was wrenched from the page by an irritating scratching noise.
Thinking it was his dog, he looks up to find himself alone, and investigating, he finds his back door open. When his wife returns, the Judge is sitting in his chair with a shot gun in his lap, reading.
Ewell becomes visibly threatening when he begins to follow Helen Robinson on her way to work as a cook at Link Deas' home. She has gone out of her way each day to avoid the Ewell place because the Ewell children have been throwing objects at her as she passes by. Deas goes by the Ewell house and hollers in a threat to Bob to leave Helen alone, or Deas will have him arrested. The next day, Helen walks past the Ewell place and no one throws anything at her. However Bob Ewell follows her:
All the way...Helen said, she heard a soft voice behind her, crooning foul words.
Helen calls Deas at his store and Deas come back to speak to Ewell. Threatening him again, Deas threatens Ewell, saying that if he bothers Helen again, Deas will "get you in on the Ladies' Law" so he better leave Helen alone.
It is also upsetting when Ewell spits in Atticus' face. Atticus takes it well, simply wishing that Ewell wouldn't chew tobacco—which makes it an especially nasty experience.
And after the children are attacked, Aunt Alexandra feels partly responsible as she had a prophetic feeling about the children's safety, and feels that perhaps she could have done something to prevent it.
Bob Ewell commits many alarming acts during the story, but I assume you are concerned with the ones committed in Chapter 27. First, Bob "acquired and lost a job in a matter of days." He was fired from his WPA (Works Progress Administration) government job for laziness--"unique in the annals of the 1930s." Bob blamed Atticus for "getting his job," and Aunt Alexandra was worried about Bob getting even, too.
Next, Bob apparently tried to break into Judge Taylor's house. Taylor was alone on a Sunday night, peacefully reading while his wife was at church. When he heard an "irritating scratching noise" that turned out not to be his dog, Judge Taylor discovered his screen door open. He noticed a shadow at the corner of his house, but he never saw the face of his "visitor."
Finally, Bob began stalking Helen Robinson, Tom's widow. He "chunked at her" while "crooning foul words" to her as she walked past the Ewell house. When Link Deas, Helen's boss, found out, he threatened to have Bob arrested, and the stalking stopped.
"I don't like it, Atticus. I don't like it at all," was Aunt Alexandra's assessment of these events.