What is a main cause of testicular cancer?

Testicular cancer or cancer of the testisoccurs when cancer cells form in one or both testicles. These cells begin to change and grow uncontrollably, forming a mass or tumor. The cells can also invade the blood stream and lymph system and spread, leading to tumors in other areas of the body called metastases.

I believe its because cancer cells begin to emerge in the testicle