What happens is, when they get fixed, they're basically losing the ability to make estrogen, which is needed for
muscle development. If you deprive them of that estrogen
too early in their life, they lose muscle tone, or don't develop enough muscle tone, in their bladder, and subsequently, leak all over the place.
(*What the internet says: Urinary incontinence may be caused by an underdeveloped urinary system or may arise as a part of the aging process.