I think it was a slow progression. There were plenty of rational ways to support trans people, and there were and are many trans people who aren't TA. Since it got attached to LGB, and most trans people were trying to live their lives and endure dysphoria, no one really questioned it. This was back in the days when you could say "Lucy was born a boy" or "born in the wrong body" without horrifically triggering someone and being a major transphobe.
Then I don't even know when, maybe the past 10 years? Since I certainly remember a time before all this, it became morphed with a weird, pomo adaptation of queer theory and has certainly snowballed with online social justice movements which thrive on cliques and echo chambery-ness. They thrive on being a cross between 1984 and The Crucible, basically. Complete with "I saw Goody Osburn still follows that TERF we told you all to unfollow!" shenanigans.
And it kind of never got questioned by anyone other than the big bad gender critics - second wave feminists who are vetoed in favour of twitter users and Laverne Cox basically.
Younger feminists, I count my younger self amongst them, just thought they were being fair and moral. That terfs were the same as homophobes. Since twitter really does act like a source, that's why facts like terfs personally murdering seven million trans women an hour is never ever refuted. TERFs are worst than MRAs, religious fundies, governments. And the rational views just never get aired. The madness is completely normalised.
Even outside of that arena, why the trans lobby has so much pull - I think mostly because it's seen as just another civil rights checkbox? So trans people are the next lot of people to gain right, and since the KKK, anti-feminists and homophobes were all awful, everyone is being very accommodating to the trans lobby no matter how extreme because they are trying so hard to not see and not mention the facts. Especially when theres a risk of death threats, no platforming and basically condemnation/ostracism amongst friends for asking questions which sheds doubts on the whole thing. If a lot of your circle are like this, you risk losing them all. A few years ago a lesbian was accused of being a terf and other crimes. Promptly everyone was told to block and delete her from social media, not socialise with her offline, and those who didn't had to be blocked, deleted, banned from attending groups. Months later names would be posted on the facebook page of these groups with why do you still socialise/connect with this toxic terf? This extended to her best friend who was quite senior, being asked to cut ties. Of course, she didn't and neither did I.
Being called a TERF is fine for most and can have no effect, but on uni campuses for example, it's an ex-communication.