Because it's bandied about and shouted from the rooftops every time a person dares to comment on a trans thread with anything other than dazzling praise? You know, with things like facts or biology.
Say anything that disagrees = transphobic.
Say anything that is biologically and factually true that they don't like = transphobic.
And never mind the utter bollocks I saw on another thread about using the wrong pronoun being violence 
I mean, WHERE does this shit come from???