Why do a lot of thread titles have "trigger" in them?

That's the word we use when the content of the post may upset people. That is, it may "trigger" or "set off" memories or strong emotions about what's described. Using Trigger Warnings means that other people can avoid threads which may be explicit or cause issues.

We use them within posts too, if there's just one section that might be a problem. They often look like this *may trigger* with some detail here *end*.

The usage varies a little depending on the subforum and what people want in order to be safe when reading there.