I can only guess, but he may just not really be someone who can give what a relationship needs, and being in a relationship makes him feel crowded. The Love Avoidant type who doesn't ever want to feel obligated (I dated a few of those, remember.)
I guess, in retrospect, you've often spoken here about how he wasn't quite meeting your needs - I guess we all should have taken that a bit more seriously.
Yuck - this really leaves you with a bunch of questions, doesn't it? But the MOST important ANSWER is - a guy who would do this like this, is NOT THE GUY YOU WANT TO BE WITH. Don't go down your rabbit hole of childhood abandonment issues - this is about HIM, not you. You did everything right this time.