JK coulda kept her mouth shut and have remained as a beloved author forever. I’ll never understand what could drive someone to taint their legacy for zero gain.

The clues to who she is were there all along in her writing. She couldn’t keep quiet, because she believes what she’s doing is right. Because she’s conservative.
The only major black character in the series is called “Shacklebolt”. The only Asian, “Cho Chang”. Zero LGBTQ representation in the books. Harry had the world at his feet and decided to join the police. The whole struggle of the saga is for a return to the status quo, rather than a better world. General lack of female agency, and women just being hysterical and needing to be slapped out of it. Goblins as an antisemitic trope.
I put it to you that it was inevitable that, one way or another, her rancorous bile would have spilled out into the public debate as soon as she got famous.

JK Rowling was always a liberal centrist. From an old 4Chan post:

It very neatly describes the way liberals see the world and political struggle.

Lots of people complain about the anti-climactic ending, but really I don’t think it could any other way. I’d like to imagine that there’s some alternate universe where Rowling actually believed in something and Harry was actually built up as the anti-Voldemort he was only hinted as being in the beginning of the books. Where he’s opposes all the many injustices of the wizarding world and determines to change their frequently backwards, insular, contradictory society for the better, and forms his own faction antithetical to the Death Eaters and when he finally has his showdown with Voldy. Harry surpasses by adopting new methods, breaking the rules and embracing change and the progression of history. While Voldemort clings to an idyllic imaging of the past and the greatest extent of his dreams is to become the self-appointed god of a eternally stagnant Neverland. Harry has embraced the possibility of a shining future and so can overcome the self-imposed limits Voldemort could never cross, and Voldemort is ultimately defeated by this.

But that would require a Harry that believed in something, and since Rowling is a liberal centrist Blairite that doesn’t really believe in anything, Harry can’t believe in anything. Harry lives in a world drought with conflict and injustice, a stratified class society, slavery of sentient magical creatures, the absurd charade the wizarding world puts upto enforce their own self-segregation, a corrupted and bureaucracy-choked government, rampant racism, so on and so forth But Harry is little more than a passive observer for most of it, only the racism really bothers him (and then, really only racism against half-bloods). In fact, when Hermione stands up against the slavery of elves, she’s treated as some kind of ridiculous Soapbox Sadie. For opposing chattel slavery. In the end. the biggest force for change is Voldemort and Harry and friends only ever fight for the preservation and reproduction of the status quo. The very height of Harry’s dreams is to join the aurors. a sort of wizard FBI and the ultimate defenders of the wizarding status quo. Voldemort and the Death Eaters are the big instigators of change and Harry never quite gets to Vold/s level. Harry doesn’t even beat Voldemort, Voldemort accidentally kills himself because he violated some obscure technicality that causes one of his spells to bounce back at him.

And this is really the struggle of liberals, they live in a world fraught with conflict, but aren’t particularly bothered by any of it except those bit that threaten multicultural pluralism. They see change, and the force behind that change, as a wholly negative phenomenon. Even then, they can only act within the legal and ideological framework of their society. So. for instance, instead of organizing insurrectionary and disruptive activity against Trump and the far-right, all they can do is bang their drum about what a racist bigot he is and hope they can catch him violating some technicality that will allow them to have him impeached or at least destroy his political clout. It won’t work, it will never work, but that’s the limit of liberalism just as it was the limit of Harry Potter.

My initial theory is that JK Rowling saw trans women as a threat to her status quo. At the end of the day, that’s all Centrist Liberals care about. Social progress can’t affect their status quo. It’s why Centrist Liberals will always back fascism. Fascism is designed to protect the status quo. The problem is that Centrist Liberals don’t understand that fascism requires an out group to work and those Centrist Liberals will be the out group at one point.