First off I'd like to point out that it's not a trend, as time progresses society becomes more open to things and people begin to become okay with being who they are, saying it's a 'trend' sounds like you're really uneducated and shouldn't be posting here for an actual discussion.
Second, I am personally transgender (inb4 hate bandwagon) and it's not something someone does for fun or for a 'trend'. It's a serious thing that makes people hate who they are, it makes them severely depressed.
Imagine going to bed as you are now, waking up and being in the opposite genders body. Seriously, take a minute and think about having to live your life when you know you're X gender, but your physical body is the opposite gender.
It's a struggle to see the things you like, want to do, how you want to act, but can't because they're afraid of what people will think of them, and worst case scenario, possibly even hurt them physically because of it.
It's just as much of a biological mix up as being gay is. It's a thing now because we're not from the 1950's. Our society (for the most part, even though it's debatable) are accepting to everyone, so people feel they can be honest about who they actually are. Nothing changed, but people feel that people are accepting enough to actually come out and be open about who they are.
A trend? Who decides one day that they're going to be depressed and hate themselves for the majority of their life for fun? Seriously? Don't start a topic, or respond to a topic if you have that mentality. It's pathetic and you need to stay out of the discussion.
(Sorry if my statement is a bit jumbled, I'm not great at explaining things)