Hypothesis: Because trans people proves that there are no "natural gender roles". "Natural" gender roles are what is used to deny rights for people (both women and LGTBI) on the basis that "we wouldn't want that rights anyway". Example: "Women are more naturally nurturing while men are better at maths". And then, comes a trans woman and says that she was taken more seriously and her work more valued, when people thought she was a man. She is the same person, her skill and knowledge the same, yet people treated her differently depending on which gender presented herself* (English is not my language, so I might be using the wrong words to express the idea).

So makes perfect sense that misogynist are transphobes in the same way that they usually are homophobes. And there is also a really high correlation with racist. People who is racist has a high chance to also be sexist and in turn, also be homophobic and transphobic.

Also, if they manage to make feminist fight among ourselves, then we won't be fighting against sexism. So for them is a win-win.