Weird banding on Canon 7d explained!

The other night I was out filming traffic. I ended up getting a really weird camera artifact. It looks like a dark bar that rolls through the shot from bottom to top. At first it made me freak out a bit, thinking that the camera was somehow broken. I eventually figured out that it is the result of A) a flickering light source and B) the right (wrong) shutter speed. I think the streetlights were tungsten bulbs, but they may have been HID bulbs. Whatever they were they definitely had a flicker to them.

I read somewhere that ideally you should shoot a shutter speed that is double the frame rate. This gives you a 180 degree shutter and it should look very similar to film. The way that I made these bars appear was by changing the shutter speed away from 1/125 when I was shooting 720p60. I have ideas as to why this creates the banding, but they are all theories. If you have any more insight pleas tell me in the comments!
Moral of the story: keep the shutter speed at twice the frames per second. Change the aperture, or buy a faster lens!