Nobody really knows Maggie and when that is the case, instead of saying we don't know, it is invariably labelled as "auto-immune' and that is what I am doing.
In HSP the immune cells take a dislike to normal cells going about their business, not bothering anyone and tries to destroy them.
Once again it is not clearly understood why the immune cells should act this way, but in HSP it is thought to be triggered by an infection, probably viral and probably an upper respiratory tract virus.