I personally react to visuals, like seeing an image of someone cutting or watching someone playing with a knife. It's not an emotional trigger at that point, it's a physical craving for pain.
Perna is right also. For me, when I am reminded in any detail of the reasons that I injury myself it can bring my mind to those dark places and trigger an emotional response that results in SI..
It's my reaction to the reminder that triggers, not the reminder itself.