It would not surprise me being that the divorce rate is going up world-wide (where divorce is allowed that is! ;-) ). But I think in Japan, there are more and more unhappily married women being that the "salaryman" husbands tend to work so much and perhaps those wives feel alienated and lonely at times.
Another thing is a lot of women are out working and there are bound to be opportunities for all kinds of things to happen; especially in very crowded cities such as Tokyo. Japanese women can certainly be as flirtitious as Japanese men, and putting all these factors together they may long to be single again. Likewise, it is the same with men. They go out in the big city, get tempted by what they see and what is around them and get weak. Then the wife eventually finds out and it is divorce time.
Also, it is the increased influence of the West in Japan that is bringing an increase in divorce, infidelity and too much "freedom" in terms of not being commited for life, not knowing and/or not adhering to one's place in the marriage and family, etc.