I didn't used to have many good friends who were separated or divorced, but now I know several and am aware of others of about the same age who are going through the same thing.
It seems to me that a number of marriages that have struggled on for quite some time will finally reach breaking point at about the time the youngest child leaves school. It is almost as if one partner or the other has said 'That's enough. I've put up with this for long enough, my work with the children is done, and now I'm done too.'
The other thing about it is that in retrospect, the issues leading to separation or divorce seem to have been there almost from day one, a bit like a tree that starts life as a sapling with a divided trunk.
I'm a believer in the idea that marriage is for life, and that the commitment to love rather than the feeling of love is what causes marriages to grow and thrive. That is going to mean a lot of joy and a huge amount of work. What I would wish (and what I encourage couples who are getting married to do) is that people would put that work in from the very beginning, with help from others around them. The patterns of communication and miscommunication are set up very early, and seem to continue unless addressed.
Of course they can be addressed any time, even by couples who have been in difficulty for 20 years. But by that time, often one partner or the other has reached their limit, and there are 20 years of bad relating to undo, rather than just six months.
I have to keep thinking about this as I'm leading a marriage weekend later this year, and in the meantime I happen to be married.