Infection of the breast.
Breast feeding, alterations in duct drainage.
Acinomycosis may be a cause as well (PubMed).
A recent study demonstrated that lactobacilli were equal to antibiotics, they did not have a placebo group and they did not report whether susceptabilty results made a difference in outcome, so it is so flawed I remain skeptical (PubMed). CID loves to call studies "Major Articles" but I am not certain they use the same definition of 'major' that I do.