I guarantee the rate is exactly the same today as it was 50 years ago, 100 years ago and 1000 years ago.
However, a number of factors make the numbers look like they are growing. Social perceptions, awareness, acceptance, disappearing traditional gender roles, are some contributing factors.
Think about it. 30 years ago if someone admitted to being transsexual, they would be labeled as a pervert, mocked, rejected by family and friends. They either have to hide and be miserable or come out and be miserable. I mean, that still happens, I'm sure Ms clones can back me on that. But people are rapidly becoming more aware that a) someone who is experiencing gender dysphoria is dealing with some heavy crap anyway and need friends not bullies. B) mocking and chastising them is just making it worse and makes you a horrible person who is just being mean and bullying someone who is already struggling emotionally.