The "demure" trend blew up because it's basically weaponized deadpan in situations where that makes zero sense. You see someone get handed something amazing or witness something wild, and instead of the screaming-and-jumping reaction everyone expects, they just go "thank you, I'm very demure, I'm very mindful" with a tiny smile. The absurdity works because it's such a *deliberate* mismatch between the emotional temperature of the moment and how they're responding to it.
I think the earlier takes about restraint and contrast are onto something, but they're missing the actual comedic punch - it's not just that they're *not* reacting big. It's that they're actively pretending to be prim and composed while doing something that'd normally make anyone lose their mind. Like, there's intention behind the weird behavior. It's not accidental awkwardness; it's chosen awkwardness, which makes it funnier because you can see someone suppressing what they actually feel. That gap between the real reaction happening inside and what's coming out is what makes you laugh.
Plus there's a rhythm thing too. A lot of these videos are short and punchy - someone does something cool or exciting, *beat*, then the deadpan response hits. That timing pattern gets baked into your brain after watching a few, so when it happens again you're already primed to laugh. It's simple humor for sure, but that doesn't make it less effective.