Triggers can be sensory (what we see, hear, smell, taste, touch), or they can be emotional.
So, you might have just eaten but if you walk past a bakery, the smell of fresh-baked bread may <trigger> a desire to have something to eat, even though you are not hungry. Hearing a song on the radio that used to by your song when you were with a now gone lover, could trigger feelings of sadness or loss.
On an emotional level, feeling a trigger emotion (i.e., hurt, rejection, abandonment, embarrassment, etc) may hit you harder even though the current trigger feeling doesn't have the power of the original trigger.
Seeing children playing happily on a ballfield may trigger a person because he is hit with sadness for not having that experience (or having it in the past but losing it due to abuse).
Anger is usually never the triggering feeling. There's something underneath that leads to anger.
For more info on triggers and how they affect you, see:http://www.malesurvivor.org/ArchivedPages/singer2.html