Don't know. I am also a molecular geneticist. As far as my knowledge goes, we still do not know how cancer develops in most cases. But we have made significant progresses. We now know the interaction between genetic factors and environmental factors is the major cause. For example, certain gene mutations can increase the risk of breast cancer, such as brac, p53, pten, etc.
Bad luck. . There are certain behaviors that can increase a woman's chance of breast cancer. As can genetics. But often it's just bad luck. Something goes wrong in the genes of a woman's breast gland cell and it doesn't get repaired. That bad cell then multiplies out of control until it takes over the whole body crushing all the normal organs. See cancer.org or cancer.gov for a wealth of information. .