Why?
What would change if you knew the answer? I've asked my wife why she's done things since she left, and without an ounce of snark or guile or hostility in her voice she's replied "I don't f*&!ing know ICE I'm crazy."
My T would probably prefer the term "emotionally labile/dysregulated," but the gist of my wife's statement is true.
Maybe he wants to hurt you. Does that make it easier to see him as he is and let go?
Maybe he's just emotionally immature and doesn't realize what he's doing hurts you. Does that let him off the hook or give you hope he can change (I have bad news on that front)?
Maybe he doesn't know why he does what he does because he's emotionally dysregulated. Then what?
I know you want to know. ALL of us want closure and to understand what the hell just happened. At some point, though, I promise you won't care any more because in the end whether he means to or doesn't or is somewhere in between doesn't matter: all that matters is how it makes you feel and whether or not you can become well enough that he can't hurt you any more. It hurts so much I am so sorry but it will get better and you will only delay your recovery - THE most important part - by analyzing why HE does what he does.