As the other millennial (early 20's) who voted in the poll, I'll chime in!
I think a lot of our generation is just really desperate to escape the daily race; go to school, go to work, go to bed, and repeat... relentlessly. We see others get caught up in this, but we don't want it for ourselves. There's more to life, as I'm sure anyone who makes a habit of exploring the great outdoors - anyone on this website - would attest to.
I do some mental health advocacy/reform work, and I always remember one of the guys I worked with saying that "we're growing up to more or less be slaves" in the sense that we're being driven to work harder and harder with less and less gain (i.e. it's difficult for a lot of my peers to buy a house or buy a car). Hyperbole or not, I think a lot of us don't want to end up in a situation where we're only working to just barely make ends meet, because that seems to be the case for some of the older millennials who've made it through college. They have an excellent education, but also a lot of debt and are faced with a lack of jobs. Many of us were brought up with this promise that post-secondary education = job + house + money. Some regions and some disciplines seem to be hit harder than others, so it's tough to say if this applies to everyone, but I certainly see it expressed by the younger folks I work with in Ontario.
Related to that, I think a lot of it also has to do with how we see our leaders (mis)handling natural resource management and the environment. It's especially scary because a lot of the things going on are so unprecedented (e.g. wildfires, floods, hurricanes, and animal die-offs on a scale never seen before, in locations we'd never expect them). There's a lot of fear about how things will turn out in the coming years, and a lot of us just really want to connect with nature while we can. Almost out of a sense of duty. I grew up next to a forest with some very rare plants that were transported here by glaciers millions of years ago. It's a unique and beautiful place, but also one that's quickly succumbing to more and more suburban sprawl. I'm sure a lot of the vandwelling and overlanding culture is a rebuttal to that. Basically, a lot of us millennials want to live in harmony with the land, and tread lightly while we experience it, rather than tear it down.
For myself, it's a mix of the above things. Most of all though, I just want to live a calmer, less connected, more peaceful life. I also think a lot of useful skills aren't taught these days. I'm a university/college-educated millenial like a lot of my generation, but I still consider those self-sufficient life skills - how to fix things, how to build things, how to grow things - far more important than whatever I learned in university. Granted, a lot of my older friends in university don't value practical skills and self-sufficiency in the same way I do (I think there's always been a culture of "by being educated I deserve not to work with my hands") but I think millenials in general are pretty inclined to do that more than other generations. My grandpa was a blacksmith and cobbler from Ukraine who moved here to start a homestead, and ever since I was a kid, I always played around in his metal shop. One thing led to another, I began welding in my free time, doing some metal fabrication, woodworking, started building my own greenhouse, fixed up my crappy first car, found an interest in vehicles, and ended up here. Overlanding lets me experience the outdoors and hands-on activities I've loved as a kid, which seem discouraged or hard to find in today's society, despite a lot of my peers desiring that.