We don't know for sure what causes cancer of any type. Cigarette smokers do get lung cancer a lot more often than non-smokers, but not ALL cugaretter smokers get lung cancer, nor do all lung cancer patietns been smokers in the past. Obviously, there are more factors involved than we can identify. For some patients there are hereditary factors, for others there may be environmental factors like exposure to asbestos dust. There are even some patients who don't have any predisposing bad habits or high-risk medical history, but still develop this terrible disease.
Breast cancer is very similar.
We do know some things that increase a woman's likelihood of developing it, but there is never a single isolated cause that can be idenitified. There are some things that increase a woman's chances of developing breast cancer. One of the most common is a family history of female relatives who had breast cancer. Others include not breast feeding, having a first child after the age of 30, or radiation treatments of the chest for other serious diseases. Most scientific studies done in the past show no increased risk for breast cancer associated with taking hormone replacements.