Why does my bunny growl and pounce?

I have two female lionhead dwarf bunnies they are indoor bunnies and i snuggle with them for a few hrs every day. recently tho i have noticed that the larger of the two growls and pounces at me with her two front paws if i try to take her sister or blankie out of their cage. I think its kind of funny but im sure that its not a good thing sooo whats going on and is there anything i can do to make her not so angry at me ?