First of all, it was 60-70 families, and second of all, WY probably sent in multiple researchers with added security to research the derelict after Aliens 3. After finding eggs, they probably started getting hosts for the xenomorphs a la Alien: Resurrection. Breeding got out of hand, shit went down, hundreds of new aliens. And if Aliens Infestation counts for anything, then the UPP will also be victims of facehuggin' love. But that's just my theory.