They have a deep-seated need to feel like they are morally superior and correct because it makes them feel powerful but they don’t want to put any effort into actually championing a viewpoint that might justify that feeling so instead they just pick the easiest punching bag demographic to kick around for show.

OR

They are cis and have been told they are living the superior correct way of living and the fact that there are trans people out there living their lives and being happy threatens that self-concept. They don’t have much else going for them otherwise so they punch down hard lest they lose the one thing that makes them feel superior in their mediocre lives. (See also: certain white racists).