This is my theory anyway.
People raised in lax environments where they were able to make their own decisions become J's.
People raised strictly with impossible standards (with consequences if they failed to meet) become P's.
The former upbringing instills confidence, decisiveness, and assertiveness. The latter deprives the individual of the opportunity to exercise their own faculties and fosters a resentment towards authority/rules.
I notice this is true with most people I know from childhood. It's true for me - obviously my overzealous xNTJ stepfather (who, in middle/high school, *required* me to study each day and to get staight A's) didn't plan for me to end up ESFP. :p