We have no idea what causes cancers, particularly breast cancer, in those who are young. We know some people carry risk factors (BRCA1 and BRCA2, certain subvariations of each) in their genes, and yet not all cases of people with the genes will get cancer, and not all people who get cancer young have any gene (in fact, most don't).
We also don't know why some cancers metastasized and some don't. Most women who get breast cancer at her age (39-40) survive.
I say this as someone who is extremely high risk (high enough that I have not hit thirty and I'm hunting for a new breast specialist who can get me MRIs instead of mammograms). I'd like to be able to know what to cut out to decrease my risk, beyond generic eat right and exercise (and I do eat right and exercise). I'm a tad angry that information is not available to me (I've looked). Because on a personal level, I'd like to stop seeing younger cancer cases.