Actually not really a new trend
It's been going on for at least 5-6 years in russia. While you call it dangerous and crazy, I'd say thrilling and daring
I am myself is kind of an adrenaline addict, unfortunately I'm afraid of heights :/ So I'm stuck on ground level haha.
I agree that those videos makes my stomach tingle though
And for getting into those structures, all of them are more or less open and easy accesible. I don't know if you ever been to a construction site of the sort, but even here in sweden, most of the doors and stuff are open.
UErs, aka Urban Explorers often uses construction sites to get to places where normal human beeings usually dont have acces to. (I know that UErs usually go to old ancient places, but still)