Tumblr is the social media site that is usually given credit for starting the trigger warning fad. A big part of the reason is that a lot of it comes from people trying not to be rude because of the way the website is set up. A lot of people having a reason to warn for completely reasonable things devolved into people being asked to warn for balloons.
Balloons are a real and unexpectedly common example.
Tumblr is designed so that you have the option to add tags to everything that you post. These tags help people find your posts. There is a search bar that works like most search bars do. You also have the option to click on a tag to be taken to a page with everything that has that tag. You also have a dashboard with has everything that people you follow have posted, advertisements, and recommended blogs.
Tags are serious business. There is a general consensus about how they are to be used. A lot of it is for practical reasons. People that wear Lolita fashion would not want to scroll through posts about the novel. If something is controversial, people might want a tag only for people that like or dislike it.
Quite a few people have something that they can use to block certain tags. I would guess that a lot more people use them to avoid spoilers, subjects that they just aren't interested in, and spam than use it solely for trigger warnings. (For example, I'm currently blocking Orange is the New Black, IDI, some spammer urls, virralift, Pitch Perfect, and bethyl.)
The search is where things get tricky for people like me that only post stuff that could be triggering. This is also where
@joeylittle 's advice would stop being too helpful. If you post a photo, there is absolutely no text. So someone searching for "dementia" is going to see my gif of a man crying and being forced to drink alcohol by his caretaker. They would only see a content warning if they decided it would be a good idea to click on the photo to see more information, and it would be rather late for it then. It would be as rude and inconsiderate as someone coming here to post random violent images.
His performance is amazing, but I guess that would probably make the whole thing worse.
Then, you add that there is a standardized format that people have to adopt for all of this to work. The one that takes the least amount of work and is advised for putting the responsibility on the person wanting to avoid content? Slapping "Trigger" on all of your posts to make people figure out for themselves if they want to see it. I generally refuse to do this because of my discomfort of the word. I do not really judge people that do, because I know that there are people that face more pressure than I do.