With the advent of Open Access, more research is becoming available to a wider variety of researchers. Further, many organizations require publishing for tenure or advancement and funding agencies require publication. Unscrupulous publishers are entering the field with the goal of making money off unsuspecting authors. These are called "predatory publishers".
These journals generally claim high impact factors or peer review while the reality proves otherwise. As quality journals begin to require authors to pay fees, it becomes easier for fake journals to trick legitimate researchers into submitting their articles to be published for a nominal fee, generally providing instant (non-reviewed) publications. Additionally, most predators these will accept any article by anyone on any topic and call it "scientific."
There are a variety of reasons authors may publish in a predatory journal:
NIH linked information on publishing in authoritative journals.
Complaints that are associated with predatory open-access publishing include (summarizing Wikipedia and references below)
Significant resources or publications (mouse over for full citation):