After clicking around a bit through Andrew's link, I found this explanation of how the Late Answers Review queue works:
The way the late answers queue works is by checking two things.
- Was the answer posted at least 30 days after the question was asked?
- Was the answer posted by a user who has 10 or less reputation (disregarding suspension) [but see below]
If both of these are true, it is entered into the review queue until it eventually gets reviewed. In most cases, this will catch things as they come in and so you will only see new posts, so we don't spend any resources checking time.
However, on rare occasion, someone will actually drop below 10 reputation, while having some past answer that does qualify. That's what happened in your example - the user received a dunk in reputation from 15 down to 5 on December 21st, 2013, and so this ancient post of theirs that has not previously been reviewed, is now put into review that same day.
Earlier today, at someone's request on Meta StackExchange, the reputation limit was changed from 10 to 50, causing thousands of answers on sites all across the network to suddenly qualify for the review process.