epochryphal:

It’s largely linguistic in reasoning!

The -ed removes agency and passivizes the modified subject.  That morpheme suggests that the subject is operated upon, transgender-ed, by something (presumably society), and is thus altered from an original (“normal” - cisgender) state.
As evidence: there is a fairly common faulty back-formation, the verb “to transgender,” from “to be transgendered.”  This is sometimes used to mean “to transition,” which is a mistaken meaning of what transgender means anyway.

This phrasing basically posits nurture over nature (regardless of the individual’s personal feelings on their identity’s source), suggests being transgender is a result of brainwashing/group-think or faulty raising or some sort of change, and leads to a pathologizing and “cure”-based approach.

Of course any individual may prefer transgendered to transgender!  I have heard some folks speak up and say that they do indeed feel society made them this way.  Completely valid.
But as a universal application, it is far less neutral and leaves trans folks less room to position themselves, and honestly gives people the wrong overall impression and gives media pundits a subtle way to continue painting trans people as passive victims and objects.

It may not be a big issue, but it definitely does influence how people think, however subtly.  Language is tricky that way!

I’ve said it before and I’ll say it again: Our greatest enemy is grammar.

(via tal9000)