I think it's about more subtle ones, or how triggers can just pop up unexpectedly and you might not realize it's a trigger until it gets you. I've not read this other thread you speak of to know the context, but that's my guess.

An example from my recent events as a "trigger that comes out of nowhere" was a candle. Just an ordinary candle, I like the smell and the flame is normally comforting, and I had lit it many times before. That time though I was bored while watching a stream so I was just holding the candle and looking at it. This apparently triggered something in me and despite feeling decent prior to holding it I started to feel all of these really bad feelings shortly after holding it. I haven't lit a candle since.