I break onto trending in sci-fi for a day or two after I post a new page. I'll make it into the overall trending for an hour, or a couple of hours, here and there ... at least that's how my last three updates have gone, since I started really looking at that stuff. And my comic is very new and only has a small number of subs and views.
I thought that Popular was decided by new subs, likes, comments and views ... and that trending was a mix of the Popular algorithm and Tapastic staff thumbs upping comics. In short, I think that I read somewhere that trending is half likes/etc and half Staff curated. Someone who has been around here for a longer time can probably confirm.
I also get the feeling that the algorithm they use isn't just based on flat statistics (total # of likes, subs, etc), but weighted statistics (perhaps something like how many likes it gets in relation to total number of subscribers, etc). But that's a total blind guess based on very little information, and I could be soooo wrong. 