Social contagion

Specifically it affects women disproportionately because women are more susceptible to social influence.

The interesting thing is the difference with MtF transgenderism which presents in two ways, hyperfeminine gays and autogynephilic fetishists

OP and u/alienhunter gave most of the reasons I'd attribute for FtM, although I'd add a fear of penetration / masculine men, and internalized homophobia / upgrading from Butch to stunning and brave straight transman.