The studies that I have found a linking abortion and breast cancer talked about a physical change that the breast cells go through in preparation for milk production. The sudden change in hormones caused by an abortion leave the breast cells of young women in a state more susceptible to cancer.
From article at: http://www.lifenews.com/oped12.html
Angela Lanfranchi, M.D., "In short, interrupting a normal pregnancy with abortion leaves a woman with an increase in the number of undifferentiated breast cells [the Type 1 and 2 lobules] that are more susceptible to carcinogens. These carcinogens include estrogen, a type of hormone found in hormone replacement therapy and birth control pills. Abortion of a first pregnancy is especially harmful and most damaging to teenagers.
Thanks. I've seen a number of posts commenting on the relationship between abortion and breast cancer, but none had ever proposed the mechanism of action.