trigger started in a blog one day, now they all use it.
triggering started because of the legitimate existence of conditions like PTSD, and the atmosphere of anti-"neurotypical" communities (which often encourage self-diagnosis and thus many people listing having disorders like PTSD) popularized it as basically a way to get people to avoid mentioning things other people don't want to hear on the grounds that it sets them off in a legitimately harmful manner, which is obviously a silly exaggeration.
and that has little to do with feminism really other than the fact that these communities were probably highly left-leaning, so naturally they would have feminist opinions