@mewmew Trap is considered a slur when used to refer to trans people for the same reasons that referring to trans women as crossdressers, drag queens, or gay men is a slur. Trap refers to crossdressers who pass well - calling a trans person a "trap" is calling them a crossdresser, which is something they're not. It's a way of invalidating their identity.
I don't really see why there's such a big push to claim trap is inherently transphobic or inherently a slur, just because it can be used as a slur. Terms like crossdresser or drag queen, which also both refer specifically to people who crossdress, can both be used to insult trans people in exactly the same way as trap, yet it seems like trap is the only one that gets much backlash currently. Though I have seen some people who claim both "crossdresser" and "drag queen" are inherently transphobic terms too. Personally I think the LGBT community as a whole is pretty anti-crossdressing atm, maybe because people are too stupid to tell the difference between trans people and crossdressers.