What's wrong with just hanging out? Granted, hanging out at those places for no reason isn't exactly the first place I'd go to hang, but I find it hard to believe your a guy who doesn't find one way or another to chill out.
There could be a lot of reasons why but I'll bet I can guess the general scenario: Depending how young a teen you are, say, 15; you're too young to drink, smoke, drive, or go to good movies (i.e. R-rated movies). Sure, you could hang out at home, but that gets boring. So, why not hang at some place public? Being a suburban kid, I know all about boring small towns, so it doesn't bother nor surprise me to see a group of kids dinking around Walmart, or walking around parking lots at 11 at night (becuz I've done that shit before myself).
So basically they're hanging out there because they're bored, beast. Kids get bored, they go do random things like hang out in the parking lot at the mall at one in the morning, and try not to get caught by douche-bag cops.
Of course I'm just talking about at night, if it's during the day and they're bored that just means they need something to do. Lots of parents won't even let their teens work, studying is lame, and if you got no hobbies and activities you do, what's left? Hang out. I'd also hardly call this a new trend, beast, if you've EVER lived in a suburban or god forbid rural area, you'd realize how boring it gets.