My parents fought a lot, my sister got all the attention growing up, and my parents basically just didn't worry about me--almost to the point of not recognizing my existence, or so I felt. So, I suppose I felt that I had to be extremely capable and responsible in order to take care of myself and other people. I feel pulled to be responsible for my sister to this very day.
I was always a clear E5, even at the age of 5. Highly independent and my parents never interfered with me too much. I withdrew from everything, and lived on escapism/fantasies. I had very little connection to anyone, especially not my parents. I liked school, but I hated the other kids, they were, and still are too restless, too aggressive, too chaotic. Burning people's hair is not my idea of fun, and neither is attacking bee hives.
I come from a family which is both hyper-conscientious and highly loyal. I was shy and developed fears of loud noises and thunderstorms which have carried on to this day. My parents took us travelling (I'd been all over Europe by the time I was eight) but were also somewhat over-protective.
If enneagram is really more about nurture, those are some of the factors I'd point to.