I've had panic attacks for five years now, and went to a few psychiatrist/psychologists during that time. They too wanted to identify a "trigger", but like you, none came to mind. Mine seemingly just came out of the clear blue at the age of 29. I had/have a good job, good life, and always considered myself kind of laid back and healthy. It took awhile to accept the fact that I was having panic attacks, but when I did, it made dealing with them a whole lot more acceptable.
To answer your question, though, I never have been able to determine a trigger. I have no clue what brought them on, and/or why, but I know they're here. I just deal with them with a healthier lifestyle than before, especially as much cardio as possible, and move on. I'm sure that some people have triggers, but I'm not convinced that we all do. Anyway, I hope my experience helps yours out a little, and keep us posted on how you're getting along! Good luck.