I understand why this is happening, however there aren't that many people working on these queues, and this will definitely put them off if it never shrinks. So will there be any way implemented to make the shrinking of this queue faster, such as a reviewing event where the 20 per day limit is disabled or at least lowered (as suggested in @ByteCommander's comment)?
The first thing to clarify is that the overnight increase happened because all the old answers that matched the new criteria were added to the queue. Once reviewed, they will be ineligible to reenter the queue. So this won't be a perpetual problem; it's just a temporary increase that will dissipate as people complete review tasks.
Second, as long as there are more than a thousand tasks in the queue, the per-day limit is raised to 40. You can see several people have already taken advantage of that.
Third, Late Answers can be quickly dismissed by a single user taking an action that can be as simple as upvoting a comment. (Please don't just do the easiest action: try to review honestly.)
Finally, the queue is often empty because there aren't so many late answers in a typical day. I'm convinced that there are many people on most sites who would like to review more late answers, but can't because the queue is normally empty. People lucky enough to spot them get to late answers quickly. When the queue is filled, there are people willing to jump into the fray. We should see the number of completed reviews scale with the number of available reviews. Certainly that's been the case so far:
It wasn't my intention to cause this disruption. But now that I have, I expect we'll be able to learn quite a bit about how reviews impact other parts of the site.
The threshold, being the queue currently bigger than 1K, "has been increased" to 40 already; this is clear just by looking at the daily late answers review stats, however: How come my vote cap for "First Posts" is lower than others?;
Also, it will shrink: since when I asked about why the queue increased yesterday, it shrunk by ~800 questions already;
Assuming the current rate, it should be empty in about 4.5 days.