Well, looking at it the first time we could think it's that, but it's actually the opposite.
They always think themselves cis, they have some obvious signs of being somewhat trans but dismiss them, thinking that "everyone has that" or "it's not that important".
Then, the period when they wanna try stuff happens, and obviously they wanna try things from the other gender. And they discover that they feel better, way more like *themselves* when they do that.
And this difference of happiness from what they tried and what they are now, that is make them depressed.
So they either have to accept they're trans and go though all the steps and shit in the way, or drow that feeling at the bottom of the Atlantic ocean.
Source : uh seeing people transitioning ig ? And some second hand stories about that ? Not that of a reliable source but ig it's better than nothing-