What Causes Urinary Incontinence?

My 21 year old daughter has serious trouble holding her urine. She can’t tell when she needs to urinate until it's too late. She has no sense of having to go or no build up. When she does go it’s an immediate urge and she often does not have time to make it to the restroom. What could this be? She has not had any kids, she is not over weight and she eats fairly healthy and works out.