This symbol is comprised of two tracks. One track starts at the bottom edge of the character area. The other track ends at the top edge of the character area. Neither track runs the whole length of the character area. Now imagine a person travelling along this route. They start out on the longer length of track, encounter an obstacle, and switch to the second track in order to complete their journey. This is the situation that you have on a bustling street, where people have to change track, either to overtake someone who is moving more slowly than themselves, or to avoid an oncoming person. Therefore, this symbol is an ideograph for the adjective bustling.