The base measurement for the "popular" rankings is recent likes (in the space of 2-3 days), but there's clearly something else that happens at higher numbers that keeps certain popular works near the top even during a hiatus. I've no idea what this other factor is.
On anything under thousands of subs though, the main factor is just "did you get over 30-50 likes in a 72 hour period?" and if the answer is "yes", you'll probably find your comic somewhere in the popular rankings in your genre, especially if you filter to community/free to read.