After a few days of these shenanigans, I figured out that these daily surges of subs only seem to happen overnight. Nothing at all during the daytime. And my newest video (part 112 as of now) has an enormous amount of likes, much more than any other of my recent videos. Analytics isn't of much help, it's claiming I've gotten way less subs than I actually did.
It almost feels like someone used one of these shady sub-buying sites on my behalf. That's probably not it, but that's what it feels like. And if that IS what happened, I wonder if I should be honored that someone's willing to shell out money to get me noticed, or utterly creeped out.