Crossdressers and HSTS already appeared in 80s anime. Things on 4chan have changed in the past two decades from the times of "traps are gay", people loved ironically shitposting about that but the disgust slowly baited some into it as a forbidden fetish. You could slowly see memes like this popping up on /a/, even ages before that there was "I recognize that dick!":
There were always male anime fans who liked dragonball, berserk and gundam who argued with the guys who liked lucky star, yuru yuri and other anime with little girls, it's not so much the medium as the underground fanartist pop-culture that fetishized them and made lolicon cool to them, the underbelly of the industry and creeps who watch pretty cure which is targeted at little girls. Currently these things became so mainstream that even fucking Bella Janke had that cursed "uoooooh lolis rape correction" tweet in her meme folder that got posted here.
Why are so many of those male anime fans trooning out today? Because it became more accepted and popular in their online spheres.
The western anime fandom on sites like twitter and tumblr slowly started going hand in hand with being progressive and supporting lgbt. It started being frowned upon if you're a fujoshi fetishizing gays or a guy into sexualizing anime girls- but not if you become the guy/girl! Then you can make it about how it helped you find your identity and you will be accepted by other fans. Idol anime fans in Japan are 30 year old men who watch it for fanservice, idol anime fans in the west are mostly troon 30 year old men because it would be unacceptable for a man to be into watching little girls dance. There's also the thing that trans forums like on reddit love convincing others that they need to transition, so it's social contagion. On 4chan there's always been a strong counterculture to the whole "woke anime fan", but there people slipped into fetishes like forced feminization porn and other degeneracies.
Tl;dr: there's always been narcissists and fetishists, but the modern internet allowed them to create bubbles where their behaviour is seen as accepted and socially progressive.