Transphobia, just like any form of queerphobia, serves the maintenance of property rights and the reproduction of the workforce that generates billionaires' wealth. This is where the antagonism comes from.

Also, please don't call Elon Musk that.

Point 8

https://en.m.wikipedia.org/wiki/Ur-Fascism

Rich/famous transphobes' voices are louder because they're rich, unfortunately. It's the same problem with every other awful opinion that rich/famous people have.

I do suspect though that it also has to do with social class and thinking that they are "above".

My conjectures:

Musk: Trans kid who disowned him for being shitty so obvipusly it must be trans peoples fault and not his.

JKR: I firmly believe she's selfhating in the closet trans. The equivalent to the homophobic pastor who eventually gets caught with the choir boy. We just haven't seen the choirboy yet.

Zuck: Not transphobic, just a sociopath (at worst) who will go whatever way the wind blows.

Products of their generation. As they die off others will take their place and society will move on to other groups to target.

They are a fairly small group compared to other marginalized groups, and particularly vulnerable compared to others. There is already a lot of misinformation about them. Average people often dont run into them or understand them Well enough to make informed decisions. Its easy for billionaires and republicans to wield religion against them. They are often vocal about their beliefs. Intersectionality is also important (e.g. supporting and protecting other minority demographics' rights because it is the right thing to do, and tearing each other down is counterproductive) to them, which isnt always the case. Disappearing them sets an example and is a test run for setting up the infrastructure and means to take control. See: nazi germany with the deaf, and current marginalization by the trump administration of the trans and disabled communities.

Mostly a people to blame and for others to fear monger, it's a way to make people look the other way also, a common "enemy" instigated from the true enemy. And sense there is not a lot of trans people statistically it's hard for them to defend themselves. It's all about manipulation and control over the people

There are a lot of other good answers, but it seems worth remembering lots of poor powerless folks hate trans folks too.

Part of the reason they're coming after trans people is because it's human nature to hate or fear those you don't understand and who are different. A dark, and sad part of human nature, but part of human nature nonetheless

All the other stuff folks are talking about is also a true, but it's not like they're unique in their contempt for people who are different

Correlation is not causation.

Personally I just think they're just pandering some group of people who have real power to determine how money flows in a society and they happen to be right wing conservatives.

US democracy is collapsing and the future throne is probably already set and they might have inside information.

Like you said, trans people can't really do anything. Perfect target when you want to show where you are planning to stand.

They may not be politicians but they spend money to get themselves into positions of political power as these few days have shown with Musk. And these people get most of their power from the masses that are dumb enough and bigoted enough to be swayed by a shared common hatred of groups to scapegoat for everything.

Because there are alot of ignorant people in the world afraid of what they perceive as different.

In your first two examples, regardless of not being politicians it's clear that by helping put politicians in power they benefit, so whether they genuinely care or not, it's just about money and lack of compassion to them. And continuing to drive class warfare continues to benefit them.

In your last example, I think that person is just in the ignorant and afraid of change category with an unfortunate amount of exposure.

Most people agree with the ultrarich on this issue (at least initially, before social media insanity), but only the ultrarich can afford making arbitrary people hate them without any good reason. That's why it looks like only the billionaires are doing it.

Then they go to Twitter with these opinions and go insane and the whole thing enters a neverending tailspin.

Elon is slightly different insofar that one of his own kids is trans. So that's not entirely due to Twitter, but there's also some lived experience at the bottom of it all (I assume here that he spent time with said child).