Can someone tell me how people get testicular torsion?

Unclear, but... Real reason is unclear, but fact/experience tells it occurs in men with inborn poor fixation to the sac so to form so-called bell-clapper deformity of testis. While its peak age around 15-19, it may occur at any age. Based the timing of its occurrence, it has been construed that hyperactivity of cremasteric muscles of spermatic cord may be prone to incite its twisting.