That is an honest question, and if someone has an answer, I would really like to hear it. I could understand perhaps using a trigger warning if you were going to post a graphic account of rape, but I see it being used before even the most basic casual discussion of the subject, and one that includes absolutely no details. That doesn't make logical sense to me.
OK, I'll take you at your word that this is a genuine question, so I'll try to provide a genuine answer. The trigger warning tag is not used only by feminists and is not used only for discussions of rape. It is generally applied to any discussion of a strongly emotional or traumatic nature, such as depression, cutting, incest, suicide, childhood sexual abuse, bulimia, etc. Like putting a [NSFW] or [Spoilers] tag, it's a simple courtesy to let the reader know that the author is going to be discussing something of a potentially controversial nature. Like with those other tags, everyone has a different idea about what it means, so something that you might find very innocuous, someone else might have a very strong reaction to. Since they don't know, people tend to err on the side of caution, just like they tend to do with spoilers here. It's just about making some attempt to understand your reader's situation and having a little empathy for them, that's all.