I've heard sooo many different reasons... LOL I smothered him. I didn't give him his space. I cut him off emotionally. (Doesn't that sound contradictory??) There was too much damage to our R already. He just wanted a D so we could start over. He just did this to change things for the better. I was perfect, it was all his fault.
I stopped listening to the reasons some time ago. I think even he doesn't know what was driving him. His words were too confusing anyway. So I just paid attention to his actions. What worked, what calmed him down, what brought him closer to me, etc. And I learned to tell the difference between times he was spouting nonsense because he was upset, and times I was getting a genuine response from him.
I honestly think there's a point during the sep where the WAS is just trying to come up with reasons to continue to justify their position. I just kept validating, and sooner or later, he ran out of things to complain about, and started to tell me I was wonderful again...