Why does milk flake?
I think the right word for it is curdle. Have you read, for example, the Wikipedia article on curd? It says:


  “Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria or yeast) will also naturally produce curds, and sour milk cheese is produced this way”


or this excerpt of the “Milk” page:


  When raw milk is left standing for a while, it turns "sour". This is the result of fermentation, where lactic acid bacteria ferment the lactose in the milk into lactic acid.


So, milk naturally contains bacteria which turn it acid, and hence it curdles spontaneously. You see the beginning of this process.



An invaluable source of information on this topic is McGee’s On food and cooking. Great reading if you like chemistry and food.