I don’t think this is new. I think kids are seeing other people do it in the media, and more parents are open to it. I think the kids are gay, and possibly think being the opposite gender makes it easier, and some of them just want to wear the girl or boy clothes, and their parents are allowing it.
However, the entire concept of “gender neutral” environments makes a difference. For instance, your son may see a doll, or want to paint his nails. If you say, no, that’s for girls (which is what I would do) he might forget about it. But if you allow it, then it continues on to ribbons, and dresses, living as a girl.
Parents simply don’t tell their children NO enough anymore. There are also parents who want a different gender child, and there are parents who think this is cool and trendy. And of course, there are some people who really believe they are born in the wrong body.