Recently I have been asked by a journal to review an article as a referee. I did it within 3 days, and the very next day another mail came asking me to review another article. I was reviewing it when, aft after 2 days, the editor sent me an email saying I don't need to review it anymore. What might be the reasons behind this?
The editor hastily selected reviewers, and after the fact encountered information or 'came to a realization' that having you review the article would constitute a conflict of interest. This may be real or perceived. I believe, however, this would be a less likely scenario than those suggested in the other answers above.