Why does traffic start?

I have constantly went back and forth about why and how does traffic start. Now I am not talking about the obvious reasons it occurs (i.e. accident, construction, inclement weather, car trouble) I want to know about that traffic that starts for NO DEFINED REASON. All of us have a story and my fondest memory was in Dec. 2007 in Atlanta. It was like 10 o'clock at night, my friend's and I had just finished having dinner for our friend's bday. We were thinking about hitting the club because the radio announced a couple of celebs were in town (someone is always in ATL). Anyway, I was the driver and I was coming down I-85 South when all of sudden I hit tons of traffic. We ended up being stuck in traffic for 2 hrs or so.
We celebrated the rest of my friend's bday in my car. We were all pissed and the whole time we were given no explanation. Did I mention my car was running on "E" yeah. No exits to get off at, no shoulder to pull over to find out what's happening...nothing. Not even a d---- traffic board in sight. People got out of their cars turned up music...people ran around passing out beers. It was crazy. So every time I get in traffic, I have flash backs to this day. As a result of my traumatic experience(s), it has furthered aggravated my road rage. So can some please tell me why traffic starts? Argh.