This, this, this.
I have a pretty genuine trigger regarding stalking--specifically, stalking and fetishizing people being portrayed as romantic, because that's exactly what Nick did to me. Shit like that causes me to go straight into panic mode. And stalking would be a reasonable thing to hope people would tag for...but what makes me especially panicky is, of all fucking things, ships. As in fictional pairings. There's a specific Homestuck ship that is frequently shown as "cute" or "sweet" among fans, despite it being portrayed as a stalker-y situation that the girl wanted no part of in canon, and that one freaks me out so badly that I can't see fanart without wanting to vomit. It causes me to flash back to everything that fucker did and said.
So yeah, I'd argue that it's a pretty genuine trigger in the classical sense for me, and while I recognize that it's an irrational and ridiculous response, I'm also not sure how I'd be able to expose myself to a goddamn ship in a controlled environment. But I'm also fairly sure that most people who dislike the ship don't have the response I have, and just don't care for it for whatever reason. I've got plenty of shit I don't care for personally, but I don't expect everyone to tag for it.
And while I do ask people I'm friends with on Tumblr to tag that ship, I can't--and shouldn't--expect that everyone will. It'd be nice if people did, but saying "literally everyone in the fandom MUST tag this ship because ONE person has panic attacks over it" is unreasonable.
Sorry for rambling, but yeah. Part of the issue is that the term "trigger" has become, as others have mentioned, meaningless on Tumblr, which is a problem for those who have actual triggers. It's the same reason I think nounself types are harmful to the tranny community. I think that if someone's first encounter with something serious is with someone treating it like some cutesy accessory (which is what Tumblr does with triggers, IMO), they're gonna be less likely to treat it as a serious issue.
(For what it's worth, I think the shift towards calling things "content warnings" instead of "trigger warnings" is helpful, especially since some things could reasonably squick people out without being a personal trigger for them. And the concept of "squicks" in fandom is one that needs to come back into vogue, IMO.)