Hmm I also agree with the PP who said woman are divorcing in their 50's now. That didn't used to be a thing; now it is. I'm late 20's and most of the friends I grew up with; their parents have divorced/split in the last decade. I know it's not all cheating; only one girl's parents had that, but the dad worked away a lot and I guess it just happened.
Most of our mothers just got sick of infantile men behaving like an overgrown child after their children had long since flown the nest
Certainly in my scenario, and talking to old friends at weddings etc; the drive seems have have fallen on the mother and the reasons have been that the husband's behaviour is no longer diluted amoung 3 or 4 people, the husband has behaved like an overgrown teen and not done anything for himself/sulked and whined/made life difficult, or they've just grown apart.
Our parents married in their late teens and early 20's, that's 30 plus years together. People are also living longer and realising after their kids leave home when they're late 40's/early 50's, that they could still (touch wood!) live on for another 30/40 years quite happily. The thought of remaining with that person is too much, for some.
Divorce is also easier than it used to be too. My mum and my friend's mums have made a decent amount in the 90's with their respective partners in the housing boom; so when the time has come to divorce, it's also financially viable. Lots of them have come into inheiritance, which may have paid off the mortgage and then had money left over. They can, even after the costs of the solicitors etc, when everything is split down the middle, go and buy a nice new home somewhere else and go and live a happy life.
My hometown is a small place in the South of England, not really near a major town like London or Bristol or anything. Quite countrified and regular, nothing special. So hardly city dwellers 
I don't think these women are immune. Marriage is not a commodity to be smug about. You either have one, or you don't. You can work as hard as you like; but as soon as one person stops working at it; that's it. If you don't both want it; it's done.
I'd actually say my parents were pretty smug about their marriage going back about 5 or 10 years. I never realised the full extent of what went on behind closed doors, and how much unhappiness was hidden. Yet my parents would both tut at other's divorce news like 'such a shame... you've got to WORK at it...Thrown it ALL away...'.
My Nan has always said along and happy marriage is much like the secret to a long and happy life. There isn't one. It's more luck than judgement 