Red-eyed dust bunny
Maybe unattended spheres are attracted to the tarrasque. Something to do with the beast's gravitational pull, or the fantasy equivalent.The sphere eats up the part that went in it, and then the tarrasque regenerates it. It is a very big beastie, remember. You'd have to move the sphere very fast in a somewhat complicated pattern to get all of it in time.
Perhaps that's why the tarrasque is always so pissed. You would be too, if you had a swarm of black specks constantly sucking up bite-pieces of you as they glide along their eccentric orbits. When it sleeps for months, or decade, it's because a particularly vital piece was eaten, or perhaps one of the spheres simply came to rest in its brain, where it sits until dislodged by some earth-shaking or cosmological event like a comet passing overhead or a quake.