1. Some people genuinely feel they are or should be a different sex. Misgendering evokes extreme dysphoria which is painful and upsetting.
So misgendering reminds them of reality which can't be changed no matter how much they want it to.
2. Some people may pin all their future hopes and dreams on being accepted as the target sex. Misgendering destroys that.
So misgendering reminds them of reality which can't be changed no matter how much they want it to.
3. Specifically some may have hopes of being loved and finding relationships and acceptance as their transitioned sex. Misgendering faces them with a different reality.
So misgendering reminds them of reality which can't be changed no matter how much they want it to.
4. In some cases there are similarities between gender identity issues and personality disorders. Not surprising as it’s connected with core identity/ personality. When a personality disordered person or addict is faced with having to change it can feel like a total annihilation of self. Misgendering may evoke similar feelings of annihilation, confronting them again with a different reality to that which they are trying to construct or have constructed over many years.
So misgendering reminds them of reality which can't be changed no matter how much they want it to.
5. Not sure about this one but have to include it as possibility - AGP. If a person is bonded to themselves as a female image, misgendering can threaten the credibility of that. A bit like a heterosexual man finding out his wife is really a man, that could make him upset and angry. If the authenticity of the AGP female self is threatened that could cause similar reaction. Also possibly parallel to a narcissistic injury.
So misgendering reminds them of reality which can't be changed no matter how much they want it to.
When I started this post, I thought I would be writing a different comment after each point, but it soon became apparent that the same comment was appropriate for all of them.
I would dispute what you've said in point 4 though. It seems that 'gender identity' couldn't be further from 'core identity/personality'. My core identity and personality are not threatened by anything that other people say about me. 'Gender identity' in all 5 of your points is all about how other people react to and behave towards the transgender person. In other words, it's all about being accepted/seen/treated as the person they want to be. It's not about who they are inside.