I also thought I was the only one that did that before I saw your thread

For me I think the reasons I used to cut when I'm hyper were from anxiety and the feeling of wanting to slow down and feel in control, mostly of my thoughts that are annoyingly fast at times. Another reason might have been boredom. When I'm really hyper I'm usually on my computer, playing video games, listening to music, and writing a song and still feel a ridiculous need to multitask even more.

Maybe some of your reasons are similar. Personally I find it hard to figure out the reasons behind my SI. I don't know if you're the same way, but if you are maybe you can recognize some of the things I mentioned, they might be similar to you.