Not known . Mostly docs believe varicoceles start forming in puberty from faulty valves in the scrotal veins. The blood backs up and causes the veins to swell and become blocked. Nothing you did caused them. Like varicose veins in your leg. See http://www.Mayoclinic.Com/health/varicocele/ds00618/dsection=causes.
Here are some ... Varicoecle is an inborn venous valve defects of spermatic veins & happens to some 15% of men with >90% of cases on left side and some 10% involving both right & left sides. Its real significance has been debated & still incompletely understood. Most cases need no surgery, but 3 conditions including its possibly related infertility, symptoms like leg varicose veins, & testis atrophy in adolescence.
Abnormal vein valves. Most veins contain valves which prevent venous blood from flowing away from he heart. Varicoceles are an abnormal diatation of pampiniform plexus, which are spermatic veins located in the scrotum. These veins lack valves so that there is a column of blood reaching above the heart & into the neck which exerts back pressure on pampiniform plexus of spermatic veins & cause reverse blood flow & stasis.