A lot of theories being bounced around in here, with not much to back them up.
I've had this happen to me a bunch of times, but never which such incredible results, so well done for having such an engaging comic OP.
When you get a spot at the bottom of the homepage, you actually come up at the bottom of comic episodes in the app, featured and discover, which explains the insane number of new views and subs. But I don't think you should expect these subs to be as engaged in the future as the ones who found your comic more organically. In fact, a decent number of them will unsub now that the tap's turned off.
It's plausible that some editor at Line just likes your comic and was able to slot it in to whatever theme they were going with at the time. It looks like it's cats this week. Maybe if they see you've gained enough new subs they take you off, or maybe they were going to take you off anyway, I don't think there's a way we can know how that process works as it doesn't appear to be regular.
As for how Line Wetbtoon makes it's money, this isn't a secret, and it's nothing shady. It's a subsidiary of the monolithic Naver corporation, and LW is just one of a few platforms it's investing in. They've openly stated that their goal is to tap into the US market and build a large database of users there. Clearly, they see webcomics as a way to do that, and if you can't figure out how Naver might be able to make money from building a large database of (comparatively rich) US users, then you're probably just not using your imagination.