The natural inclination of scholars to want to share their insights, as well as the increasing pressure on scholars to produce in greater volume, creates a natural opening for unethical publishers to exploit authors. Although the problem has grown since the advent of online-only publishing, it is not a new one, nor is it limited to open access publishing. In fact, questionable publishers exist on both sides of the open-access/toll-access (subscription) line.
The best defense is simply for authors to be critical of "too good to be true" publication offers. Check out Think, Check, Submit's video on choosing the right journal for your research:
We offer a variety of tips for assessing a publisher's quality.
You may also be interested this article about identifying high-quality, sustainable OA publishers from Duke's Kevin Smith.