Heres my take on it:The predalien gets loose in the ship,the ships scanners read pred and alien DNA so it asumes its a pred with a trophy(or something like that),the predalien gets big and starts killing preds.Knowing they cant take it down(or maybe they wanted to keap it alive)they leave the ship.Except the guidence system gets damaged and it heads for last known landing(subsequently crash-landing on earth),the star-drives fail two-thirds-way through flight so it then take three years to come to earth.The preds track star-drive radiation until the point where the other ships drives failed,they then hunt for the ship and find out that it has crash on earth.they then dispatch Wolf to clean it up before it infects the planet.