It's not a mistake, it's just like when you tell the creature to bite something and walk at the same time, the creature will drag it. The same happens with the physics system and the UFO, the UFO can abduct stuff so it creates some kind of gravity system to abduct creatures but if you turn it off in the middle of the process the creature will be at the mercy of the planets physics system interacting with the force the abducting beam was applying before, etc.