I think that perhaps what sometimes happens is that someone who is transgender feels aggrieved because they have been refused access or being challenged for using women/ girls' school loos, beauty salons, changing rooms, refuge etc. They may express their distress online and find people there with both the political influence, media connections, social media power, legal expertise and/or finance to take up the claim.
It may also be the case that some activists deliberately seek out opportunities to challenge the policy in one place to set precedent elsewhere.
It is not dissimilar to how individual experiences have been picked up by some from religious lobbies who have then provided the backing eg the B&B owners who sought to refuse a gay couple This may have also been the case with the teacher who was disciplined for his use of pronouns in a class with a child who identified as transgender. (I can't be sure but this was my impression when he was interviewed on the daytime program with Philip Schofield?)
The comparison with the women with cancer diagnosis (upthread) is that she would also describe her distress / offence at being refused treatment but this would not prompt a powerful lobbying 'machine'.