Why the term is "transgender" and not "transgendered"
Here is some yellow paper.
We say it is yellow because that’s what color it is.
Simple enough, let’s move on.
Here is some white paper.
We say it is white because that’s what color it is.
Now, imagine some stuff happens to the white paper - something spills on it, it’s left in the sun in a hot car, whatever. It is acted upon by some force, and now it looks like this:
We can now describe the paper as yellowed. It was originally white, but something caused it to become yellow. Yellow has happened to it - it is yellowed.
Describing someone as “transgendered” does not make sense, because that person has not been transgendered by anything. Transgender did not happen to that person, they just are transgender.
Just like you wouldn’t describe a black person as “blackened” - they weren’t originally some other color until something came along and acted blackness upon them, they just are black - you shouldn’t describe a transgender person as “transgendered”.