I think most transphobia is based in fragility of masculinity , white fragility , homophobia, misogyny. What these have in common is they exist to defend prevalent system of hierarchical classism It offends those who succumb to heteronormativity that some people don't want to be heteronormative. "White supremacist christian patriarchy is so great right? How could anyone not love it? They must simply be hateful bad people!"

Feminists get this response, mlk got this response, homosexuals and bisexuals got this response. Now trans people. All offended prevalent power structures and so faced subjugation and castigation.

Gender is a fundamental power structure.

The idea that gender is not fixedly in accordance with outwardly visible biology undermines the prevalent gender structure.

The idea that gender can change over time undermines the prevalent gender srructure.

The idea that people have a right to be referred to as the gender which they identify as undermines the prevalent gender structure.

Etc.