"Trauma triggers" is a psychological term that has been bastardized recently. They're certain kind of stimuli that bring about serious and uncontrollable emotional reactions in individuals with a history of trauma, particularly those suffering from PTSD. To be subject to trigger warnings just is a sign of mental unhealth, which is why traditionally therapy has been to heal trauma victims by controlled exposure to triggers, and not mere avoidance. Trigger warnings are designed to help those especially vulnerable to certain stimuli avoid them in general while, ideally at the same time, learning to cope with them through controlled exposure in counseling. A rape survivor, for example, taking a college course in philosophy on Sexuality or Love and the like, might wish to excuse him or herself when a discussion of consent arises because s/he only feels comfortable discussing anything related to sexual violence or violation with his or her therapist while s/he works through his or her issues. A trigger warning might not be necessary though, as a glance through the syllabus would surely suffice. That being said, the trigger warning in this case is not the responsibility of the educator, however, but the victim, in so far as s/he is responsible for warning the educator about what triggers him or her. The bastardized use flips the responsibility: educators are responsible for warning all students---victim or not, known or unknown---about potentially triggering material.

The problem with this usage is that it becomes flippant for two reasons: 1) when the desire for a trigger warning is not made for medical reasons, but for fear of emotional discomfort (perhaps of the sort people feel when watching an extremely uncomfortable scene in The Office, or a horror movie) and 2) when the demands of trigger warnings make unreasonable demands upon educators and content providers rather than the victim.

The first problem is just a case of a slippery slope. If a non-rape victim takes the same course described above, but has particularly strong feelings about the topic of rape to the point that s/he considers reasoned disagreement about consent to be something so abhorrent and morally reprehensible that she feels physically upset by the discussion, then s/he might feel entitled to claim, on the grounds of emotional distress, that s/he has been "triggered." Is her claim to "emotional distress" as strong as the actual rape victim's? Some people say yes, some people say no. There are clear cases and there are unclear cases, and we have to decide as a society how uncomfortable we have to warn people they might become upon entering certain discussions and environments. When the slope has been gone down even further though, and cases of desiring "trigger warnings" are even more frivolous, lots of people end up feeling that it has been fully misused, bastardized, and trivialized. Hence, the joke above.

The second problem concerns the demands that frivolous trigger warnings puts on others. You could do things one of two ways for that class: either a student who is a rape victim or is especially affected by language of sexual violence (putting aside the question of why s/he would be in the class in the first place) should warn the professor of his or her condition, and arrange some kind of alternate form of assessment, or whatever arrangement the two parties agree on. The second way of doing things is for the professor to elaborately warn all students about "potentially triggering" material with "trigger warnings," wholly unaware of who in the class would actually be affected by it. The first way, I think, is ideal, and shows maturity and judgment on the part of the student. The second is useful as well, at least for gratuitously disturbing material. A professor of mine, for example, showed a horror movie in class, and warned us all and very understandingly explained, "if you don't like or can't handle horror movies---which is totally fine---you don't have to come to these lessons and you can do this alternative assignment instead." To me, that's not really a "trigger warning"---since it's not in a psychological context---but it's certainly a disclaimer, and those are entirely appropriate in like circumstances.

The flip-side is when people are wholly aware of the content and demand to then be warned about it. Recall the recent incident with Christina Hoff-Sommers speaking at Georgetown University. There, a popular feminist scholar with decidedly unpopular opinions among many feminists was demanded by students to issue "trigger warnings" for her talk, entitled "What's Right (and wrong) with Feminism." Again, everybody knows what this woman believes already, but some students insisted that she add a "trigger warning"---really a "disclaimer"---claiming that her lecture might potentially upset some students. This gratuitous use of trigger warnings exemplifies both problems I mentioned: the slippery slope and the unreasonable demands on others. On the one hand, exposure to opinions you don't find appealing is why you should be in college in the first place, especially a top liberal arts college. Merely finding some opinion "unsavory" shouldn't be grounds for avoidance. Much less, on the other hand, should it be grounds for having those who you disagree with---without having heard the actual arguments for the opposing position---warn you that you "might be disagreed with," which is the only purpose the trigger warning of Hoff-Sommer's talk would have served.

tl;dr - it's complicated. there are reasonable uses and there are gratuitous uses. there are confusions about what emotions and opinions ought to be respected or discounted. there are confusions about what demands to put on individuals and institutions. My personal opinion: educational institutions would be better off if people didn't use it so much, and the responsibility shifted more to the students.