First off I think that we can all recall doing things that were not so nice, and there were also times when we all wanted to bolt, but we didn't because we made a promise and a healthy emotional adult doesn't just walk away from a promise. Hey it wouldn't be a real marriage if everything were perfect afertall we are all human.
I also know that you're very right about trouble brewing wayyyyyyyyyyy before the onset of the bomb. That's a lot of the problem. While these exes are hatching a plan, but not yet courageous enough to make it a reality, we may see subtle changes, but there again to us this is a no brainer. When we took our vows it was for the good and the bad, so possibly we just see it as a period of discontent or maybe something they're going through. In all fairness to us, how could we know the extent of which to take this, you can't change what you don't know and since they weren't being upfront about their feelings we don't take it seriously and think it will pass.
When we are finally filled in to how serious this is, we are confused and somewhat scared so we become the fixers, and singlehandedly take this upon ourselves to make this work.
All of this becomes easier when we finally understand that the only one we can make happy and fix is #1.
The biggest flaw in this whole scenario is that we think because these weak people walk it somehow means we were not enough, that is so wrong! At times, it's by far so much tougher to stay than it is to leave!