I think it’s because a lot of her fans were like “hang on, she’s just misunderstood, she’s not really transphobic, just doesn’t want those fake transes going into women’s spaces”, and Rowling kept it pretty vague, open for misinterpretation, not really ever able to be pinned down as actually being provably transphobic in the minds of many.
But something happened recently and it tipped her over the edge, from keeping her donations low key and her transphobia aimed at the theoretical “not really a woman ‘trans-identified-male’ boogeyman-woman” (who doesn’t really exist), to outright making open donations to anti-trans groups and attacking real trans women.
All the sudden her supporters are like… Wait, this is not who we thought you meant… Where are these scary men? The ones invading women’s spaces that you were talking about?