When I bought milk, I found that the box was swollen, indicating that the milk was broken and I couldn't drink it. The most likely reason for the bulging milk carton is ()

The bulging of fruit juice and milk beverage boxes is caused by the accumulation of fermentation gas produced by a large number of microorganisms. These microorganisms thrive in the nutrient-rich residual juice in the box. Drinks with long shelf life are usually heated sufficiently during processing to kill harmful microorganisms.