C.O.
I just gave birth to a baby (my third baby) with a 2-vessel cord two weeks ago. From what my drs. told me, they have no idea why it happens and it is quite common. It CAN be an indicator that something else is wrong, but not necessarily. I'm glad everything worked out for you and your baby.
On a different note, I'm SHOCKED at how little your dr. did to monitor your baby!!! They had me in for monthly bio-physical profiles (ultrasounds) until they noticed my amniotic fluid level kept dropping. I then had to go in weekly, and then twice-weekly because it was getting dangerously low, ESPECIALLY for a 2-vessel cord baby. They would have done them weekly anyway to make sure things were OK. Even if the cord is not an indicator of a more serious problem, it requires extra care to make sure other complications do not stem from it or cause stress.
I was also induced a week early because the amniotic fluid level drops naturally towards the end of pregnancy, which can stress the 2VC and cause complications. My dr. said there was no way they would let anyone in that situation (carrying a baby with a 2-vessel cord) go past 40 weeks.