Suddenly popping up a modal dialog is one of the most popular solutions nowadays to get likes and subscriptions on a website. The dialog might request to fill in a form and share the page, subscribe to a newsletter, or use to some sale offers.
Such trick might drive up likes and subscriptions, but it is a real user experience killer. It is simply annoying. Annoying because the modal is unexpected and not triggered by the visitor. The modal auto-triggering is called to block the workflow to make a fundamental decision or to display a critical error. In this case, subscription forms or ads placed in the dialog. It is a bad practice to auto-trigger the overlaying dialog in term to serve the marketing goal. The correct way is to serve people with the best user experiences. If the task of a business is to annoy people, rather than helping to achieve results, it is a merely unethical business.
Suddenly popping up dialog destructs the visitor from the task appearing with a timed delay in an unexpected moment. It obstructs the visitor from reading the content, watching a video, or performing an action.
The first reaction is to close the dialog promptly. In most cases, the visitor misses and clicks on Like, Share, Subscribe buttons. Because, those buttons designed as a Call to Action elements, the most visible items on the modal, the person misclicks on such thing. The comparison for such abstraction could be as if someone walks on the street and a stranger steps into the path by blocking the further away. The reaction will not be to Like and Share, but to stress.
Interface designers responsible for the correct use of components.
Thus, they should disallow the elements misuse and ensure smooth user flow. There are always better, non-intrusive ways to advertise or collect visitor's information. The responsible, creative person should find a better solution. It is way too easy to isolate the user and stress him into an unwanted action.