I'm 99% certain they're handpicked by an editor.
I've been on the homepage spotlight about 4 or 5 times now, and once on the Discover featured section. That time I was on both at the same time and that was wild ride. My comic went from about 500 subscribers to over 5000 over a very short period of time. From what I recall, it felt like it lasted about a month, although surely that can't be right. They do seem to tick over more quickly these days, but honestly, as someone in charge of changing banners on a site, my suspicion is they get changed whenever the person responsible gets around to it.
The homepage is a much better spot because it means you'll come up on the app at the bottom of even featured webtoons, so in that instance you can expect your regular monthly views to go up by at least 500% or more.
How to get on there though? My suggestion is to regularly put up as top quality content as you can manage and to do something different from the crowd. As far as I'm aware, very few people are doing a style similar to mine in webcomics right now, so I have that going for me. Other than that, my guess is as good as yours.
I think whatever the reason you get picked though, if your thumbnail doesn't perform, they'll probably be more inclined to switch it out for one that does, and that might explain why popular comics reappear in these promo spots regularly.