Simple minded idiots hate trans people simply because they are different to them. Trans people existing and having rights and such does not affect their personal lives.
So they know these days it's harder to be so open about that. Now they have to come up with ridiculous reasons about they have a right to hate gay or trans people. Now there's an "agenda" and it's a "choice".
The trans conspiracy is probably the most ridiculous one yet. They think that people would so uproot their lives by transitioning (something that I imagine is incredibly difficult - especially if family is not supportive) solely so they can sneak into a woman's restroom and perv out. Who would do that? The internet is a vast ocean of all manner of depraved pornography. Why the hell would they go though all that just to perv on women in a restroom?
Obviously there is something much more compelling behind their decision that ties directly into their identity.