I think most of the preds are at the beginning.Every things going fine,the ship is on course,a pred does a routine check on facehuggers & eggs,but when he thinks the one is dead he anylizes it and sees that its dead,so he opens the chamber and FWOOSH!!.Its actualy alive!,and because hes not wearing a helmut,BOOM!,he falls over and a few hours later the predalien bursts out!.it climbs towards a vent,gets inside,and reaps havoc on the ship,it then crashes on earth,the nearest planet,killing most of the preds,and the predalien breakes the other chambers releasing the facehuggers and trys to start a new hive!.But thats just my idea.I might be wrong.