What do risk factors for breast cancer such as early menarche, late menopause, & nullipara have in common?

Hormone. Longer exposure to higher levels of estrogen hormones is suggested, but these statistical findings are a measurement of increased risks. The statistics are not meant to tell us why things happen, they only measure what is seen in the us population. See gail risk model measurements.
Breast cancer. Prolonged exposure to endogenous (internal or self) estrogen.