Random white stains on teeth in back. why?

Decalcification: White stains are typically decalcification. There is a possibility of fluorosis. See a dentist to examine in person and talk to you about what you may want or need to do.

Decalcifications: The white stains on molars that form by the molar gum line usually form when we are young and do a poor job brushing and flossing. They are actually the weakening of the enamel being cause by acid forming bacteria in the mouth. Not quite a cavity, but is the beginning of one. Keep your teeth clean and healthy by visiting a dentist.

Not random?: While the distribution may appear random, spots on teeth are usually there as a result of something that happened to the tooth (before or after it erupted). It could be most likely demineralization (softening of the tooth which causes a chalky white stain); it could also be fluorosis (staining from too much Fluoride intake), or another development related exposure. Ask your dentist next time u go.

Hypocalcification: If you had a fever during the formation of your tooth, it could make them look like this.

White spots: White spots can be areas of demineralization, which is the beginning of dental caries or if they have been there your whole life they can be hypocalcification, which is developmental.