One major contributor to sperm DNA fragmentation is oxidative stress. Oxidative stress is a strain on the body caused by the built-up damage caused by atoms with unpaired electrons—free radicals—that is not adequately neutralized by antioxidants. Sperm DNA fragmentation is associated with infections, cigarette smoking, drug use, exposure to environmental and occupational pollutants, advanced age, varicocele (enlarged veins inside the scrotum), illnesses with high fevers, elevated testicular temperature (laptop computers, hot tubs), chronic diseases such as diabetes, cancer and cancer treatment, poor diet, and obesity.
Sperm DNA fragmentation also occurs when the body lacks the ability to repair breaks that occur in DNA, and when there is a deficiency of proteins that typically bind DNA and keep it in a compact form that is less susceptible to damage.
Source:
. https://progyny.com/education/how-to-test-for-sperm-dna-fragmentation/