Why did my doctor have to clamp and cut the cord during my delivery? Did I do something wrong?

No. This occurs when the umbilical cord is around the baby's neck and cannot be easily reduced or slipped over the baby's head prior to completion of delivery. There is nothing you did to cause this.
No. If the baby's umbillical cord was wrapped around the baby's neck, the doctor sometimes has to cut the cord in order to deliver the baby. Neither you nor the doctor did anything 'wrong'. Sometimes the father is unable to cut the cord.