I don't agree there even is a 'trend' as such.. Its certainly in the wind, there has been such a huge boost in awareness of trans issues in recent years that have helped a LOT of us to dare come forward and be ourselves because the places we live are growing more accepting of our situation by the day. What we are seeing is not people trying to be cool by coming out as transgender, we are seeing the people previously too afraid come forward as they recognize they are far from alone, and the world is more accepting of them than ever.
I absolutely love that in just a few years my country is now a place where teenagers frequently dare start their transitions, and get so much love and support in return.. When I came out 8 years ago this was not a reality, never mind 20 years ago! It is like a dream that my relatively early transition at age 25 has started being considered 'late', as people here are so accepted and so understood for these issues that doing it at 15-20 is now completely normal.
And if this is a 'trend', let it roll on cause its only furthering acceptance as far as I can tell..