Steve was caught completely off guard, he had no idea that the person he'd just past would be violent and while the music in his ears wasn't loud enough to block much for him, the whipping sound of the knife was too soft for him to hear through it. He get the blade dig in to his shoulder, right on target and he pitched forward with a grunt of pain. He caught himself before hitting the ground and quickly spun back toward his attacker, crouched low to the ground. One arm moved to grab the knife and pull it out. He knew he'd bleed more but it'd give his body a chance to start healing and it'd give him a weapon to defend himself.
As soon as he really looked, he knew that uniform. But he also knew that it wasn't Bucky. Everything was wrong, height, posture, shape. His mind raced, wondering if it was some trick the island played on James and it triggered him back to the soldier or if maybe there had always been more than one.
Either way, he was in trouble. He held the knife in his hand and took a defensive stance, holding his ground but not moving to attack. Not until he knew who this was.