I have an off-the-wall theory here that the time with her family strengthened her to do what she needed to do, which is why she told NUA that she was going to need her more than ever. I think possibly she told him the marriage was over and he went off in a rage. He had talked her into a third pregnancy even while having an affair because he believed that would make it impossible for her to leave him no matter what he did. When he found out that she was no longer going to worship at that empty shell of an alter, he lost it. I don't necessarily believe this is exactly what transpired but it's a possibility.
I have a very similar theory about that night. I think she had spent that 5 weeks in NC, trying to figure out if she thought she could reestablish herself back there, if they separated. I don't think she wanted to divorce, but it was becoming obvious to her that he had checked out of the marriage already.
So she got some new business contacts going, talked to a few close friends/family about possibly returning back to the area with the children and for the upcoming birth...and got used to the idea of possibly becoming a single mom.
Her husband came to visit for the week, and she noticed that he was distant and disconnected from the kids. So she knew it was inevitable that they would be parting ways...
On her 2 day business trip, she told her best friend that she was going to need a lot of help and support soon, and she decided she'd start making the necessary changes when she returned home.
At some point, that fateful night, they had their 'emotional conversation'...AND SHE SEALED HER FATE THAT NIGHT---She told him that she was taking the girls in the morning, and going to a friend's , while he figured it out. And in the meantime, she was taking the girls back to her parents, to get the help and support she needed.
That was a big problem for him. He couldn't let her leave like that. It would totally ruin his future plans. He had it all figured out that he was going to make them 'vanish' one night and then he could come home from work and say 'where is my family?'
But suddenly she was threatening to move out the next day and he wasn't ready...so he had to just go for it...