The observation that a planetoid is slowing down in its orbit could reasonably lead to the conclusion that the planetoid is being attracted toward another massive object. In orbital mechanics, a body's velocity can change due to the gravitational influence of nearby objects. If the planetoid's orbit is altering in a way that suggests it is slowing down, it may indicate an interaction with an unseen or more massive object, such as another planetoid or a larger celestial body, pulling on it gravitationally.
Other options, like being repelled by something, may not adequately explain the observed behavior in a standard gravitational framework, and friction is unlikely to have a significant effect in the vacuum of space. Therefore, the most reasonable conclusion in this scenario is that the planetoid is being attracted toward another massive object.