Where have they come from?  They've always been with us - it was just that, until recently, this was something to be ashamed of and hide (much in the same way that being gay was in the past).

Further, the medical community has begun to come to grips with what's going on and now recognizes that some children are gender non-conforming.  Children do self-identify by gender at a very young age (as early as two or three) and it's not something that can be influenced or changed.  Gender non-conformity is not something that can be "cured".  All parents can do is accept their children for who they are and be as supportive as possible.  Among other things that means not forcing them to be something they're not based on what's between their legs.