I always thought trigger warnings came from tumblr, one of a number of ideas cooked up by teenagers trying to reinvent existing things from scratch. It's just content warnings, which have obviously been around a lot longer, but framed around trauma triggers. So like, making something that's useful for everyone out to be an accessibility tool, which opens it up to a bunch of criticism and debate that was never applicable before. It would never have been a "culture war" issue if those kids hadn't needlessly rebranded and moralized the basic act of warning about possibly objectionable material.