On inspection, it looks like many "late answers" from years ago have been added to the review queue.
Is this a change to, or an error in, the algorithms that select which answers get added to the queue?
There were more than 500 entries because the criterion for sending late answers to the queue has changed.
Before the change only late answers from users with reputation up to 10 had their answers sent there; now the threshold is 50. With this new criterion, many old and late answers posted by users with reputation ranging from 10 to 50 were sent retroactively to the review page.
This is the Meta reference pointed out by JNat:
Can we raise the bar for reputation for late answers to bypass the review queue?