Okay so I've finally picked up a copy of Whipping Girl, roughly 1/3 through, and already I'm shocked that I haven't heard much of the theory discussed anywhere else even in the many years since it was published???
There's some very concise theory that explains why transmisogyny exists, though it hasn't been spelled out in so many words yet. It breaks down sexism into two main types:
Oppositional sexism, the belief that male and female are mutually exclusive: that they are fixed and opposed roles with a divide that cannot be crossed.
Traditional sexism is what most people likely think when they hear the word "sexism". It is the belief that masculinity is superior to femininity, in any and all forms that may take.
These, together, explain so much of transphobia in general as well as transmisogyny specifically. Transphobia is well explained as pure oppositional sexism, because being trans flies in the face of oppositional sexism. If someone believes that The Only Two sexgenders are rigid and that moving between them is impossible, of course they will take issue with trans people who, in popular consciousness, are solely defined by the fact that we cross that line (or simply express a desire to do so). This affects all trans people regardless of 'direction' of transition or more specific identity.
This also explains much of the hatred for non-binary and intersex people, whether they identify as trans or not. While obviously not the same as each other, both non-binary and intersex people inherently question the validity of the binary required by oppositional sexism.
Transfems, who cross this divide from masc towards fem, fly in the face of not just oppositional sexism, but traditional sexism too. By transitioning at all, we mock oppositional sexism (as do transmascs), but because the shift is from a position of man/male to woman/female we also fundamentally question the validity of traditional sexism too. The insecurity of others at having both these pillars of their gender entitlement questioned at once leads to a specific kind of bigotry aimed at the trannies who do so; this is transmisogyny. I'm not discussing the specifics of intersectionality here as this
I like that Serano frames things in this way because it highlights the actual issues here. By explicitly placing the motivation and blame on two forms of sexism, she allows an easily understood - yet nuanced - cohesive theory of transphobia and transmisogyny. This also makes it clear that no one is trying to claim that transmascs or cis women don't face any oppression, so hopefully its a little harder for TERFS to co-opt in bad faith.
I don't really know how to end this and it's messy cos I wrote it in like 3 mins but uhhhh go read Whipping Girl it's incredible so far. If anyone at all has any notes on this I'm 1000% down to talk about it at any hour cos trying to find irl people to talk about it with isn't going too well lol.