Does divorce occur if one swears an oath of divorce to threaten his wife should she leave without his permission, and then the sworn-upon event takes place?

This phrase is an allusion to divorce. As long as you did not intend divorce by it, then it does not occur. If you intended something specific and threatened your wife with it should she leave without your permission, then you owe an expiation for a broken oath by Allah if she left and you did not do what you threatened her with. We advise your wife to fear Allah and obey you in what is right, and not to leave without your permission unless it is a necessity.