Continuity mistake: When Carter is singing karaoke, first you see the girls beginning to join him on stage. In the next scene (right after the scene with Inspector Lee's reaction to Carter singing), you see Carter dancing with a DIFFERENT group of girls (it lasts a couple of seconds). Then, when Lee is calling out Carter, the original dancers from the first scene are back. Use the tall Chinese girl wearing the pink dress on the right side of the screen as reference in the first scene, then she disappears in the second scene and reappears in the last scene. (00:06:45)
Answer: Although it's never shown, it was probably Ricky Tan's henchmen. Lee and Carter were outnumbered and decided to give up. After the fight, they were dropped off in the middle of a busy highway with no clothes on as a way to add insult to injury.