I don't think there are any hard & fast rules and different consultants seem to have differing opinions. My own GP told me that ORN can spontaneously halt/settle down. I've also been told that ORN can spontaneously start although there are usually triggers the most obvious being trauma like tooth extractions, but also infection (hence the need for mouth hygiene and need to keep plaque down), periodontal disease and even cleaning (especially gum pocket/root cleaning). I have heard of people having intravenous antibiotics after mouth surgery/tooth extractions to help prevent ORN and I've heard of other people who have been put on oral antibiotics long term to stop/prevent infection.
Its a very frightening condition - best wishes, Hela