That is, an "established wisdom" among sociologists etc, that after seven years you've become so familiar with your partner that the grass starts looking greener outside the fence. (cf the saying, "familiarity breeds contempt".)
If a lot of people were being decent about this -- i.e, trying to resist the urge -- or just slow to react, or succeeded in keeping their affairs secret for a while...
In short, if there is a "seven-year itch", but there also are all kinds of reasons for a delay between its onset and its effects, then there could well be a bump in the curve over relationship-breakup frequencies at about nine years.
Just my speculation, of course; more or less a WAG.