That's the million dollar question. There are hereditary factors (like BRCA) and environmental and lifestyle factors and just plain randomness. If a woman doesn't have a genetic predisposition, it's hard to know exactly what caused her cancer, because it can be a complex interplay of many factors -- hormones, menstrual history, breast density, environment, lifestyle, and bad luck. I know I followed all the recommendations for healthy living and still got breast cancer at age 49, but I also had extremely dense breasts, frequent irregular cycles, and no children. The best we can do is get tested for genetic mutations, try to live well, follow our treatment plans, get screened, be aware of changes in our bodies, and avoid obvious risk factors like hormone replacement therapy and smoking. This link from breastcancer.org discusses risk factors.