John Gill 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)

So the children of Benjamin saw that they were smitten
Their forces broken and worsted, many being killed:

for the men of Israel gave place to the Benjaminites ;
at first, and made as if they were afraid of them, and so fled before them, which was only to decoy them to a greater distance from the city Gibeah:

because they trusted unto the liers in wait, which they had set beside
Gibeah ;
that these would not only enter the city, and burn it, but meet the Benjaminites fleeing back to it, when they should turn upon them and smite them, and so cut off all that remained of them.