Good question! It's not being trans itself that's the cult. It's the gender identity ideology...which both trans and non-trans people can believe, and both trans and non-trans people can oppose.
It's turning something which is, at its core, just an unusual (but not terribly rare) mental condition afflicting a small percentage of the population into a Big Thing, like it's some huge civil rights crusade. It's elevating the concept of "gender identity" (which is something like a sexed soul) into the realm of pseudoscience, like it's some actual real thing, rather than just a shorthand way of describing the state of being afflicted with gender dysphoria or not.
Some of the people who buy into this are doing it because they're bored, want to think they're part of some big movement that's On The Right Side Of History, and so they latch onto this because they can't find any big movements. Others are doing it because they're suffering from gender dysphoria themselves and have some preconceived ideas that people suffering from mental conditions must be "crazy" (possibly something they've heard from their own friends/family), and so they understandably want to join a movement who tells them they're fine and the problem is with everyone else. Others are doing it because they're just bad people who like bullying others and trying to get them canceled for speaking out against their group's orthodoxy.
There are many reasons someone might join up with this ideology, but regardless of the reason, it has no basis in reality. Gender dysphoria isn't that different from, say, anorexia. We should help people suffering from it with compassion, but we should not validate the worst impulses of these conditions with praise, nor should we pretend like it's some big civil rights crusade.