What is the generalization of why the milk curdled?

Milk curdles primarily due to the coagulation of proteins, typically casein, which occurs when the milk's pH drops or when it is exposed to heat or acid. Factors like the presence of bacteria, which produce lactic acid, or the addition of acidic substances such as lemon juice or vinegar can accelerate this process. This transformation leads to the separation of curds (solid) from whey (liquid). In summary, curdling is generally a result of changes in acidity or temperature affecting milk's protein structure.