Maybe you should look into genes involved with cell adhesion GO biological process. But be carefull with breast cancer molecular subtypes, they have probably different way to induce such mechanism.
There are "no reasons" for a cancer to metastasize, from breast or from any other organ.
Cancer is an anarchic and very heterogeous tissue composed of multiple different cell populations. Some of them are able to resist anoikis when they detach from the primary site and then are able to metastasize.
You shoud read the articles and reviews by Ian Fidler on this subject.