I am somewhat familiar with this, and the explaination I have been given by the TS is that the TV partner 1) reminds them of what they once were and hated (being in the middle of the genders) and they find seeing it in someone else disturbing 2) When they transition they want a "nomal" (in the literal sense) life, and just like a GG the TS is afraid of their SO being discovered to be a TV and everything that comes with exposure... PLUS their background pre-transition being discovered as well.
A TS friend of mine once felt this way, and told me years later after being involved with several "real men" for a few years she said she didn't like alpha males after and she said she liked having a "softer" man (a TV) after all. Go figure!
Again, that is what I have been told, not what I say or believe, just what I have been told.
