The question got a vote to close. Thus it was put in the review queue, so that others can express their view whether the question should be close or stay open.
The reason why it got a close-vote is, in all likelihood, what Martin said in a comment. The question got bumped to the frontpage by a new answer and thus got visibility. (The first vote to close came after the answer.)
More generally, it is not uncommon that old questions get votes to close and thus land in the review queue. Some users even look through old questions actively to see if there is something that should be closed. But this was not the case here I think.