All of that was explained in many, many books and comics. In the Avp comic (which was originally what the movie was going to be loosely based off of) you learn that the Predator race seeds planets with Alien eggs to later hunt them. I believe the planet was called Ryushi (or something like that) in the comics. Weyland Utani (sp?) the same corporation mentioned in the first 2 Aliens movies was colonizing the planet.
The hapless settlers start noticing things are amiss and discover the Alien infestation only then to discover that they are caught in the middle of the Predator's hunt. The Predator's came to Earth in the first 2 movies to hunt people, not Aliens.
This movie has been on the discussion plate for at least 10 years now. I'm going to go see it, I'm just saying they should have stuck with the "original" source material. Take Alien 3 for example. The original plot would have been much, much better than the piece of crap that made it to film.