What are causes of testicular torsion?

Bell-clapper testis: Some men have a testis that hangs loosely within the tunica vaginalis which is membrane around testis. Torsion can occur if the membrane is attached unusually high on the spermatic cord, often with a testicle which lies transversely rather than vertically. Testicle thus hangs like the clapper of a bell. Additionally, contraction of the cremaster muscle can then cause testicle to twist on it's cord.

Twisting of cord.: Testicular torsion is caused by twisting of the spermatic cord, which interrupts the blood supply to the testis. A common contributor is an anatomic defect called the bell-clapper deformity, where the additional mobility of the testicle predisposes it to twisting. A larger testicle is also a risk factor.