Well, in Algodoo, boxes and polygons in general don't have a distributed mass, instead, they have a center of mass which is the pivot point for the simulated gravity formula. The clown has a positive attraction so it interacts with every entity arround, including those without attraction. If you pay close attention, this simulation just looks like a very elliptical orbit of an object orbiting another very heavy object.
HA!
[Edit] The wooden square's center of mass is literally on it's center, explaining the clown's orbit
