Why does milk form curds as it turns sour?

Answer: Acid reduces the repulsion between milk protein particles

Gas bubbles push milk proteins together into lumps — Gas bubbles are not what makes sour milk form curds. Acid changes the stability of the casein particles, letting them clump together. A food can form bubbles without forming a protein curd.

Cooling turns the milk's dissolved sugar into solid lumps — The curds in sour milk are mainly aggregated protein, not crystallized milk sugar. The relevant change is increasing acidity, rather than cooling alone. Refrigeration slows bacterial growth but does not turn sour curds into sugar crystals.

Acid reduces the repulsion between milk protein particles ✓ — Acid makes the casein particles in milk less able to repel one another, so they aggregate into curds. Bacteria can produce this acid as milk sours; adding an acid can also cause coagulation. The key is destabilizing the particles, not simply unfolding the proteins.