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.