In Chapter 27, three things happen that foreshadow Ewell's attempt to harm Atticus and his family. The first thing that happens is that Bob loses his job at the WPA for laziness. The WPA stands for Works Progress Administration, which was part of Roosevelt's New Deal that employed Americans to carry out public works programs during the Great Depression. Scout mentions that Bob Ewell openly accuses Atticus of getting him fired, and found himself forgotten as Tom Robinson. (Lee 332) The second thing that happens is when Judge Taylor is reading on a Sunday night, and he hears something scratching near the rear end of his house. When he gets up to check it out, he catches a glimpse of a man's shadow running away. The third thing that happens is that Bob Ewell attempts to threaten Helen Robinson, Tom's wife, while she is walking down the public road. Scout says that he "chunked at her," and she tells her boss Link Deas. After Link Deas warns Ewell to leave Helen alone, Bob follows her to work the next day, keeping his distance and cursing underneath his breath. Link Deas tells Bob that he'll report him if he hears about Bob bothering Helen again.
These three events depict Bob Ewell's slow advance in enacting revenge on those who wronged him during the trial. We know that Bob Ewell is a coward, so his half-hearted attempts to scare Judge Taylor and Helen Robinson are not surprising. After he loses his job at the WPA, the first person he blames is Atticus. It is safe to assume the next person he will attempt to intimidate or harm will be Atticus. He's already spit in Atticus' face, so it can be surmised that his next attempt will be to physically assault him. One cannot predict that he will attempt to harm Jem or Scout, but the reader can predict he will go after Atticus again.