Interstate Brownian Motion

I spent most of my afternoon catching up on my programming labs. One of the bonus problems was to have a bunch of objects (cars and trucks) bounce around and to implement simple collision detection. The solution I came up with is weird and mesmerizing, particularly when you run it with 500 particles. For some reason all of the cars quickly get shunted into the bottom right corner and stay down there while the trucks float around up top. I wonder why that is. Edit: Oh and before I forget, here's a link to another project I did for the same class.