What might be some reasons to ask to stop reviewing an article by the editor?

Another review was submitted that left the editor in no doubt that the paper should be rejected (typically for reasons where there is no room for a different opinion, e.g. incontrovertible evidence of plagiarism, duplicate submission, a fatal flaw in the problem definition, etc).

Another explanation: The editor forgot that they had already asked you for a review a short while ago. Realizing this and not wanting to burden you twice in a short time, they paddled back.

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.

The most likely explanation: The editor invited too many reviewers, with the expectation that most of them would decline. Other reviewers submitted their review before you, so your review was no longer needed.

It is like an overbooked flight.

There are many possible reasons. Perhaps the author(s) withdrew it. Perhaps the editor made a decision somehow, though given the time, probably a reject.

Of course, you can ask the editor, and you may or may not get a real reason.