I agree with posters who have suggested that the real fear is fear of encountering reality.
'#bekind' has a lot to answer for, as it has given transpeople the impression that they are universally accepted as such, and that they are convincingly good at it. Neither is true, and while being kind is A Good Thing, the old saying about being cruel to be kind applies here:
Making transwomen aware [not in an aggressively not-kind way, but with clarity] that they are still easily identifiable as biological males, and that they are not and never have been entitled to use the women's toilets would have been the kinder thing in the long run.
Giving them the idea that they really are women, that they are indistinguishable from women, and that they are entitled to us women-only facilities has set them up for this inevitable and awkward collision with undeniable realities about biology and society.
Some claim to be reacting with fear, but street demonstrations, violent threats and ostentatious use of women's toilets suggests that anger, misogyny and spitefulness are more widespread than fear.