Why do rabbits hump stuffed animals?
Rabbits hump stuffed animals and other such things (like furry boots) for the same reason that humans masturbate: to satisfy the sexual urge.
"Mounting" is a different behaviour than "humping" but they are sometimes confused by human onlookers. Humping involves mounting but also there's a clear back-and-forth action and it's sexually-based. Mounting, however, is often related to dominance or social status -- the dominant rabbit will mount the subordinate rabbit just so everyone knows who's boss, and it's not necessarily a sexual act.
It's recommended that rabbits are neutered/spayed. Neutered/spayed rabbits usually won't hump things, although they may still mount other rabbits in a display of dominance. (There are many reasons to spay/neuter rabbits, not just so they'll leave your stuffed animals alone. See the related link below).