This question could mean the success or defeat of AVP2. Since it is set around the present day, I agree that it could be from the crashed ship. sounds lame... but, think of it this way.
Predators know SCAR has the chestburster, But chose to let it mature for a good hunt. It would be silly to think the Predators could not tell SCAR was host. Here are y 2 ideas...
1. Predators have to much honor to let thier own become a host, so if a Predator accidentally becomes host, then they have a special hunt in honour of the fallen(SCAR). Since SCAR was a teen, perhaps his teacher or father has the honor of the hunt. Hence 1 Pred on earth. The Hybrid unexpectedly is a Queen or can do what the Alien in Alien 1 did to Dallas.(in deleted scene).
2.The Hybrid matures and escapes into the ship, days have passed( I hated that in AVP 1 the Aliens lifecycle was like 10 min. As some Predator search the ship, others, while in earths orbit, decide to head down for a hunt or 2. While a predator steps into his shuttle to drop to earth, the half grown hybrid make a rush for the pod. It slams in as the doors are shutting. The shuttle has already begun it's prep and proceeds to drop down to earth. 1 Predator sees this happen and rushes to another pod. by the time he gets down to the other pod, all thats left is the ripped apart body of the other predator. Thus you get 1 Predator on the hunt.