What does the reference to Israel's doing evil signal?
Judges 4:1 (ESV)
1And the people of Israel again did what was evil in the sight of the LORD after Ehud died.
This reference, which is the exact same wording as the opening of the Ehud narrative, marks the beginning of the third cycle within the main section of the book.
1 And the people of Israel again did what was evil in the sight of the LORD after Ehud died.