It comes down to what abandonment means. If they leave the relationship, they were in control and made a decision. If YOU leave or divorce HIM, then YOU are in control and telling him that he is not wanted. That is crushing! That is abandonment.
I have another theory that some do threaten just to hear us give them reasons/promises to stay. I started this theory when I told my husband during a fight to 'do what you have to do' when he threatened divorce. It stopped him in his tracks. No reasons to stay, no promises of me changing to please him. Just, you are an adult, do what you have to do.
Then, I got tons of reasons we should work on the marriage, from the man who'd just threatened to divorce me. Because, even if he threatened and he filed, if I was OK with divorce, it still felt like he was being abandoned by me. This is just my theory.
The last part of my theory group about why they threaten to leave is that some don't have a good understanding of consequences, especially when highly emotional. It's like a blind spot. If I keep walking out, I'm in control of this relationship. What they don't get is: If I keep walking out, eventually, she may shut the door behind me and say good riddance! If they get that, they're far less likely to keep with the revolving door or threats to leave.
One question, that day when he walked to the post box but turned around, what did you say as he left? Were you angry, relieved, quiet? Had you told him to do it already? Because those answers, during his walk, might have sparked the 'I'm not really in control here' thought. That would support my theory that our being OK with the divorce will stop them straight away.
Your theories make sense.
The day he took the papers to the post box I said nothing: I did not chase him, did not ask not to. I went to bed.
The interesting thing was he made me believe he posted the papers. I only found out that he did not when I got from work the next day. The unsent letter was on my desk. He later blamed me for not caring because I did not try to stop him.
So I guess you are right, they push so hard and try to control us but when they see that they are not winning this battle, they start to pull back. It's all about control and validation needs.