What could cause my scrotum to descend and ascend rapidly in the shower?

Temperature change: Testicles require constant temperature for peak function. When hot shower is taken, the scrotum relaxes so that the testicles are away from the body which provides heat, and this is a means to get rid of heat. Scrotum descends. Cold water will reflexly draw the scrotum closer to the body to keep the testicles warm. This is when the scrotum ascends.