>What is material that is supposed to capitalize on surprise?
It's a hard one to answer but I'll give the answers I can from my time in university. It's hard mostly because each instructor handles the material differently so often examples are oneshots.
With regard to sociology and anthropology, the Nacirema are often presented as a real tribe at first. Many students will realize what's happening, but the point of the assignment is to encourage students to realize that from the outside even their own culture is strange and in this way, so the idea goes, they are better able to view things from a neutral position because of this lesson on cultural introspection.
Other examples could be things like literature readings of classical texts. Many students are uncomfortable saying the N-Word, for obvious reasons, but censoring it diminishes the significance of the word and while that's not a bad thing in day to day life it's nonetheless bad if your goal is to impart on your students a sense of what weight that word carried in its historical context.
Movies are another good example. There are lots of classic movies that get shown to illustrate a point but part of the point is a conclusion that's arrived at over the course of a movie. On my mind when I say this is the short movie about a teacher who basically invents racism by telling her class children with one color of eye are better than the others and then later she reverses this decision. In both cases the children treated the inferior group overall worse and none of them liked being the inferior group.
>These things are rarely core issues of lectures, triggers based on what I read can be completely random things (not actual depictions of rape, but smell, sound color) which may make them useless for most.
This is ultimately the core point. Triggers are nuanced and often specific to the individual. There's no easy way to know in advanced what might set something off and so encouraging trigger warnings is ultimately encouraging censorship of long practiced content (whether that censorship is outright or in the form of encouraging a hopefully less confrontational presentation of the content which diminishes the emotional impact of the lesson).