I totally agree with all of this. I should be using a front brake, I sort of said this in my OP. This is not a question of whether I should have a brake (I should and this incident proved it) I was just asking for thoughts on the mechanical explanation for my chain derailment as to avoid this in the future.

Also just to be clear, I did NOT almost get killed, I did not feel that my saftey had been even remotely threatened, it was like 11:00 at night and there was very little traffic. The only reason I even tried to stop is because there were 2 cars trying to turn left. I did not almost die. However, I fully understand that this could have happened at a much more inconvienent time potentially causing me harm. It was a good lesson to learn.

I tightened up the chain a little more today and tightened the crap out of the nuts. Drivetrain seems lovely. I will install a brake soon. I don't have to use it if I don't want or need to. But I just might. I also agree about being able to go faster with a brake. More confidence to bomb the **** out of ****.