One way I can think of that si might be used as avoidance is that since we anticipate some kind of pain anyway (probably of the emotional variety, which hurts worse - disapproval, rejection, abandonment, the pain of dealing with past trauma, or any number of other things), we go ahead and cause physical pain. Then we can worry about that, and maybe think about the whole issue of stopping the si habit, or hiding it, or even just enjoying it, and be distracted from the other pain. It might be a way of keeping yourself busy so that you don't think about the other.

I also use si to get out of situations I don't want to be in, like getting lectured. Because somebody can't keep standing over me lecturing me while I'm scratching and tearing my skin. That might be avoidance too. Escape, at least.