How about the possibility that they are tired of their partner? The irrationality might come from the fact that they are bound to them (e.g. marriage) and feel pressure to not break that apart. That'd be quite an internal conflict, and might produce irrational behavior.
If this is a possible explanation (I've seen it many times at least), do you really think it's an only 1% of the time cause? I think that's being unrealistically optimistic. Because then your narrative sounds very convenient and comforting: "Oh, I just have to help them get over their insecurities and it's happily ever after." Not to be a downer, but I think people need to be more realistic about these things so they have better odds of finding a partner with which they have a good dynamic.
(It's kind of like being stuck at a job you don't like, but you need the money. That's bad for both the employee and the business.)