It looks like it's staged/scripted on that video (nothing wrong with it just stating how it appears to be).
I also love girls who can't hold themselves. Real urinary incontinence is a medical condition though.
I think KANON and Erena (really old video) in Gachinco/10 Musume really peed themselves since they did it on several occasions. Other than that, it's a very rare thing to happen. A hard and long fuck on a girl would increase the chance that she takes an involuntary pee - think of a long orgy or gangbang.