Torsion is not a common problem. It happens in about 1 in 4,000 males under the age of 25. It can also happen in newborns and in older men.
In most males, a testicle can't twist because the tissue around it is well attached. Some males are born with no tissue holding the testes to the scrotum. This lets the testes "swing" inside the scrotum (often called a "bell clapper" deformity).
Torsion can happen on either side, but rarely on both sides. Physical activity doesn't cause torsion. It may happen during exercise, sitting, standing or even sleeping.