The third picture is the closest I could find online, but I am wondering more about the first two pics.
I’ve been calling it either the “Kim Jong Un” or the “Miller from Expanse” look. I keep seeing men ages 18-30 or so having their hair like this. Shaved on the side with a slick of sorts on top.
Is there a reason this is becoming a thing?
Here is a pic of Miller from the Expanse and no, I haven’t seen it this extreme.
Undercut
See also, Fade:
https://theidleman.com/manual/mens-hair/best-fade-haircuts-men/
Just a random trend, IDK if anything precipitated it, but the undercut is getting to be known as an alt-right thing and the fade is just generically trendy.
Also known as “mushroom hair”, specially when longer. The guys I’ve known who wore it like that did so because at work they were required to have a cap on and all (long) hair had to be covered; the shorn part would be outside the cap, with the long part covered by it. I expect it’s copied by others who just happen to like the look.
A cousin of it, the sidecut, is also getting trendy for women, especially in Sci Fi (you can think of it as the “Naomi from The Expanse cut” 
)
There’s also the Marines “high and tight” haircut, where the sides are nearly shaved but they are apparently allowed to have it a little longer on top. Funny, when I googled that, one of the photos was the third one in your first link. Anyway, so maybe some Marines just keep this look after they’re out and let it grow a little more on top.
Really common with kids in SE Asia. Got my grandies’ hair cut in the same fashion in Vietnam in May. Dead simple to do, and it’s somewhat cooling.
We’re not alt-right or anything though. 
That third style just looks like classic hipster to me, which has been around for years. It evokes the ‘short back and sides plus brilcream’ styles of yore. Team up with some fancy whiskers or excessive beard for full effect.
There’s never a reason for any trend. They just are.
Like Jragon said, the alt right took it over, so it’s on its way out amongst “normals”.
It comes from the alt-right?
Indeed, some people have taken to calling that style “the fashy” (from “fascist”). See this article in the Washington Post, or just do a Google search on “fashy haircut.”
Edited to add: It doesn’t come from the alt-right–as the Post article explains, its roots go back to Victorian England–but it’s been adopted by the alt-right recently, which sort of taints it for more reasonable people.
Ive been doing a kind of fade thing for a while. Short on the sides and close to a faux hawk on top. I combine that with a full beard/stache that mirrors the hair. Short on my cheeks, blending to a six inch or more beard.
My barber came up with this. He cuts mostly black hair and likes to play with white hair like mine as a learning experience.
Also note, I’m pretty sure that Naomi and Miller’s hairstyles are a call back to the “Belter crest” that Larry Niven used in his Known Space books. When you consider a life lived where you might need to put on an emergency vacuum suit at a moment’s notice, a hairstyle you can stuff into a helmet, but which still looks good without the helmet, becomes a practical matter.
Undercut was popular in the 90s but definitely not as refined as it is today. Back then it was basically "pull your long hair up in to a ponytail and shave underneath."Like this. Several of my friends had it but it was definitely just for cool kids and definitely weird and different. The hair on top had to be long enough to do a ponytail, and it was never styled. Maybe once in a while they’d do a mowhawk. Or they’d let one side grow out and just have a shaved half.
A variant of this this was popular where I lived in the late 80’s and was simply known as the “Skater Cut”. I got it in 89 and even then it was on the way out. Similar to John Connor’s in T2 but shorter on the sides. I seem to recall shaving it on a 3 guard. Very long bangs that when back to the back of the crown when slicked (had to do that for work, otherwise I wore it down like John.
Getting trendy? Looks like what we called a Skrillex cut in the day, which I recall ladies getting back around 2013 or so (and I wasn’t running in cutting edge fashion circles then either). Well, I guess ladies and DJ Skrillex as well.
It’s a thing with women too. Do a Google Image Search for “chick fade.”
One of the first times I noticed that look on a man was the character Jimmy Darmody (Michael Pitt) on HBO’s Boardwalk Empire. It was set in the late 20s-early 30s.
It’s also used on Peaky Blinders by a bunch of men who were in WW 1. I wonder if it was a gas mask thing?
Ugly.
