As I see it, an MLC doesn't necessarily mean there was anything very wrong with a marriage. It's my impression that it can also result when a man realizes he probably has no more time left in his marriage, life, or career, than the time he has already been married, lived, or worked. And he realizes that not much has changed in those areas in many years, and he's suddenly panicked when he realizes that he's likely to spend the rest of his marriage, work career and life with no new changes, and he's going to get old and die. Men are taught that life is limitless and there are many doors to be opened. But at this point, he suddenly realizes all the doors are closed and everything from here on is not limitless. Rather, it's totally predictable and completely limited, from here to the grave. And that's all there is, nothing more. So he comes to the sudden realization: "Oh my god, is this it? Is this really all there is to it?" A sense of panic and the irrational thought that there is still some time left to open some more doors.
I can sort of see it, though my philosophy is more practical. I focus more on what I can do TODAY. But I've heard what I've written above from several men who have found themselves in sudden MLC's.
One man was a university professor, and he woke up one day and suddenly realized he had only so many hundred weeks left before he was going to retire, and only so many months and weeks before he would be "old". The numbers didn't sound big. Indeed our lives are short. We all have only a handful of decades and it's over. But I was struck by how this professor could just suddenly wake up to this fact and be panicked. But I can see how it results in a feeling that "I've got to do something -- anything -- even drastic -- to shake things up".