When I run my testicle & penis retract into my body a little but not all the way. Why is this? Can I prevent it?

Your question. I don't think it is anything abnormal. You don not need to worry about it. The cremaster muscles in the scrotum can contract while running and cause the testicles to retract.
Normal. Thisis normal physiology. I am unsure why you would want to "prevent it"?