Chemicals causing it are produced by corinebacteria and other microorganisms. These glands secrete an oily fluid with proteins, lipids, and steroids which are odorless before microbial activity. They appear on the skin surface mixed with sebum, as sebaceous glands open into the same hair follicle. Unlike eccrine sweat glands, which secrete continuously, the apocrine glands secrete in periodic spurts.Compounds responsible for fatty odor include organic acids, esters, aromatic compounds. One example is tridecanal that has aldehydic smell (while nonanal and decanal smell musty), another is (E)-non-2-enal (fatty with a citrus nuance and additional green, cucumber, aldehydic notes).
Check also this blog with results of earlier Aurametrix studies and the comments.