The positively charged particles in acid break apart the groups of negatively charged proteins in milk, forming clumps. Caseins are largely the type of proteins found in milk.
The proteins in milk form separate groups. One end of these proteins prefers touching only fat while the other prefers touching only water. Therefore, they form groups of proteins, and the positive end, which prefers touching fat, faces inward. The other ends, then, are facing outward. The negatively charged molecules bounce off each other. The acid is positively charged, disrupting this process. Therefore, the groups of proteins, called micelles, begin to clump together, known as curdling the milk; cheese is made from the curds of milk, though the curds are usually formed through enzymes rather than acid.