> If marginalization were the main cause

I think they're crazy first, trans second. They were marginalised for being crazy. Then they found each other because they're trans. Many cults have random attributes shared by the members, whether it be race or sexual preferences. Their race or sexual preference didn't cause them to join a cult, they had other things going on that drove that. But when it came time to join one, they gravitated towards the one that identified with them.