Why does a mother cat attack me if I clean a kitten's pee?

Why does a mother cat attack me if I clean a kitten's pee?

Female cats are very protective of their kittens and the waste the kittens make do have the smell of them so this might trigger the mothers protective instinct.

the mother trust you more than the other people around for now,but this is not positive for the kittens they need human interaction to be good house cats.

you do not say if the mother cat allows the kittens to be lifted and played with by other people,it is important for the kittens to have human interaction from the time their eyes open for them to be used to people.