The cause of the increase is primarily due to the female hormone, estrogen, already known to be a risk factor in breast cancer.
Scientists also agree that a full term pregnancy helps protect a woman against breast cancer.
Scientists agree that cells are most vulnerable to carcinogens and thus to cancer during their transitional, unstable stage before maturation is complete. Once maturation has occurred, the cells will never return to their previous undifferentiated state, thus the woman's risk is reduced. Induced abortion unnaturally removes the source of estradiol and progesterone which had stared the process of differentiation. This leaves the woman with a higher number of cells in the process of differentiation than she had before the pregnancy, and leaves those cells at that most unstable state, where they are most susceptible to cancer inducing carcinogens. If she later has a full pregnancy, a woman can slightly reduce that risk, but never in its entirety.