Charles Ellicott Commentary Judges 20:36

"So the children of Benjamin saw that they were smitten; for the men of Israel gave place to Benjamin, because they trusted unto the liers-in-wait whom they had set against Gibeah." — Judges 20:36 (ASV)

That they were smitten. —The “they” refers to the Israelites. The rest of the verse gives the reason for the feigned flight.