Oof. What a topic. I've definitely encountered the term, along with other gems like 'sudden onset gender dysphoria' and 'desistance'.
The terms are not broadly used within the transgender community. They are used, and used heavily, by a number of groups of a certain political and religious bent, as a way of describing transgender people very inaccurately. In particular these terms are used to describe transgender youth in a very negative light.
A "trans trender" is supposedly someone who claims to be transgender because it is the cool thing to do, "everyone is doing it" or some similar rationalization. I have had some claim that
I wanted to transition because of this, that I had too much time on my hands in retirements and because Bruce decided to transition to Caitlyn, I wanted to also!
*SIGH*
"Sudden Onset Gender Dysphoria" is another made-up term. Actual gender dysphoria has three properties that mark it as being real;
persistent , insistent , and consistent gender identification as a gender other than that sex assigned at birth, as characteristics of dysphoria present in the individual. It is persistent, and consistent, not appearing 'all of a sudden'. The only thing sudden about it is someone finally figuring out or admitting what is going on within them.
(Note that a person, particularly youth raised in a very affirming and supportive environment may not necessarily experience actual gender dysphoria, but may still have that
persistent , insistent , and consistent gender identification as a gender other than that sex assigned at birth. When supported, loved, and assured that they will grow up correctly, albeit with a bit of medical intervention around and following puberty, they might not experience dysphoria. This is a wonderful thing!)
The terms are real, out there, and not particularly supportive of our community. I see them used primarily as a component of arguments that we are not real, and that supporting or accepting us in any way is injurious to us!
You can see the terms in use in various references this article points to, for example:
https://www.lgbtqnation.com/2018/06/conservative-medias-attacks-trans-people-become-orwellian/