The Benjamites looked behind them It is likely the Israelites shouted when they turned about to fall upon the Benjamites, which made them look back to see what unexpected supplies they had received. Then they saw their city on fire, which, with the sudden turning of the Israelites from flight to attack them, quite put them in confusion.
Related Commentaries of Judges 20:40
Judges 20:40
40 But when the flame began to arise up out of the city with a pillar of smoke, the Benjamites looked behind them, and, behold, the flame of the city ascended up to heaven.