Haha aww, my cat did this to her kittens, too! I was like, 'HEY WTH IS WRONG WITH YOU, YOU IDIOT?!' because she kept climbing up onto the dresser and dumping the one kitten (she had four, then three after one was put down due to a heart defect, but she always picked on the same kitten hahah) into the hallway (which confused me no end because the baby-gate was there to protect her and them from our dog and DD1).
Turns out, mummy cat thinks her kittens are getting big enough now to leave the nest and go exploring on their own. Or that's what I like to assume, anyway - the alternative is that she's turned into 'bad mummy' and is trying to ditch them hehehe. Try taking the kittens out of their box for short periods and let them stumble around in an open area, so they can get a taste of the big wide world Mummy cat should keep an eye on them and then take them back to their box when she thinks they've done enough wandering for one day HTH!! Congrats on the little kitties!!