BPD, Self Harm and Triggering

Like most Borderlines, I self harmed in the form of cutting for a good chunk of my life. I'd say I probably started around the age of 12 or 13 and I didn't stop until I was 29. The first time I cut, I did it because it was cool. All my friends saw what I had done and I swear their view of me went up a notch after that. I continued to cut every once in a while, but not too often because it hurt like the dickens!

Then came the day when my horrible temper showed itself. Like always, I was out of control, screaming, throwing stuff, punching the wall. Even after all this time, I can still remember how mixed up I felt inside, how badly I needed to explode. I don't know what made me think of it, but I got out my razor and I started cutting. It was the first time I had ever cut without feeling pain. Instead, all I felt was exhausted relief. That day, and for the next 17 years, I would use cutting as a form of anger control. 

Now you're probably wondering why I took the time to tell all that. I did it to explain why I cut. I've seen so many instances where a Borderline reads or hears about someone cutting and that triggers them into doing the same thing. It's like they go crazy with the need to self harm. But I don't understand why. The only thing that's ever triggered a cutting episode for me is feeling that extreme anger and not wanting to explode. For me, it's never been about "feeling the pain" because I don't feel pain, just relief. I think if I actually felt the pain, I would have never cut again. So anyways, the whole point of this post was that I really don't understand triggers. So if someone wants to explain it to me, that would be awesome!