It's multiple things, a fair amount of it is fear of the unknown.
I'm prefacing what I'm about to say by noting that I am not a professional psychologist, but I am an amateur psychologist.
In psychology there is a concept called "need for cognitive closure" which basically means that we need answers to questions about the world around us. We all have a certain level of this need, but for some people it is cranked up to a very high level. People like this will take the first explanation they find that comports with what they've already learned, and will not change their minds no matter how much evidence there is. People who exhibit bigoted behavior tend to score very high on need for cognitive closure measurements. Organized religions offer easy answers to life's questions, so these people tend to be very attracted to religions, most of which are anti-LGBT.
For some people they simply can't understand why you would want to change your body. They find the idea of a feminine AFAB taking testosterone and masculinizing to seem wrong, the same for an AMAB individual feminizing. Though what's interesting is that that very discomfort is our dysphoria.
For some it's a challenge to their sense of identity, this is common amongst TERFs.
There's also some status anxiety, the idea that a group they consider subhuman being thought of as equals feels like a loss for them. These are the people who really really have to defend calling us "abnormal"
I'm sure there are others, but these are the main ones I see as I masochistically read comment threads on trans related news articles.