Milk curdles when mixed with orange juice due to the presence of acid in the juice. Orange juice is naturally acidic, with a low pH level. When the acidic orange juice comes into contact with milk, it disrupts the proteins in the milk, specifically casein. In normal conditions, casein proteins are suspended in the liquid. However, the acid causes these proteins to coagulate, or clump together, resulting in curdling.
This reaction is similar to what happens when milk sours due to the growth of bacteria, which produce lactic acid. In both cases, the increase in acidity causes the proteins to form solid curds, separating from the liquid whey. Therefore, when you mix milk with orange juice, the acidity of the juice is what triggers the curdling process.