When you exert yourself, the pressure in your abdomen increases. If your pelvis, the floor of your body, is weakened, the urethra will slide out of place. Then the locking mechanisms that are located by the urethra and that are intended to help you hold water don't work. Your pelvis and these locking mechanisms can sometimes be weakened and deteriorate after pregnancy and childbirth. That is why this problem is more common among women.
At menopause, the amount of the female hormone estrogen decreases. This hormone also influences your body's ability to hold water. Estrogen deficiency makes your mucous membranes fragile and the risk of leakage increases.
In some cases, men can be afflicted with stress incontinence. This can be due to having been operated for prostate problems or having some kind of back injury.