As mentioned above, predatory publishers exploit the Open Access publishing model, particularly the Gold Open Access model. The Gold OA model allows the published version to be uploaded into services such as institutional repositories without embargo and authors retain full copyright control for a fee. Unfortunately, predatory publishers have seen this as a way to make money while delivering no valuable content.
A caveat: Open Access does not make a publisher predatory. There are many publications using the Open Access publishing model that are reputable and ethical. What makes a publisher predatory is their intent. Predatory publishers make false claims, such as quick turn around on peer review and falsified impact factor data, to lure authors into submitting papers.