They are too small and still blind, so mother cat cares about them. Cats defend their territory bravely, and charge into battle “with open visor”. And defending its territory or defending some of its other vital interests, cats prove it: they never attack stealthily, never try to bite or scratch back.
They always warn the opponent, making chest rumbling, which from time to time turns into a hiss. In this case the cat stands on stretched legs, abruptly bent back, its hair standing on end.
She would want to be taller, more impressive. This is one of the tricks that afford cats. And what to speak about kittens? She won’t allow any offense.