He had guessed they would need a hasty escape since he would rather not battle the Nazgûl inside the city. There were too many people in Bree and the chances of innocent bystanders getting hurt were too great. The Ringwraiths might unleash their anger at the townsfolk after the battle if the Elf fought them there. It was much better to draw them from the city before a confrontation, so Legolas had prepared for the occasion.