Why does casein precipitated as sour milk?

Casein is a milk protein that forms micelles, which are spherical structures that keep the casein suspended in the milk. When milk sours, the lactic acid produced by bacteria causes the pH of the milk to decrease, which leads to the destabilization of the casein micelles. This destabilization causes the casein to aggregate and form curds, which is the process that leads to the formation of sour milk. The curds then precipitate out of the milk, forming a solid mass that is the basis for many dairy products such as cheese and yogurt.