It sounds, to me at least, as though your husband may be having second thoughts or feeling guilty about his decision. That’s not uncommon. My husband’s ex did much the same thing as did my ex.
The thing to keep in mind is that sometimes it’s difficult for a person to grow emotionally while they are with someone. I know that no matter how much time passes if I were to ever consider going back to my ex (shudder) I would be going back to the person I was when I was with him before. Things may be different for a while, but eventually the way you are with someone creeps back in and you realize that after 3 months, things are right back to how they were before, when you hated it and longed for more out of life.
If you are changing and growing, moving past your separation and relationship, then your spouse is trying to figure out why those same changes couldn’t have happened while he was there. Sometimes it takes something drastic, like separation or divorce for us to see what we don’t like about our lives. Sometimes it takes this to have the nerve and the opportunity to begin living how we want to instead of how we always did before.
I’m not suggesting that two people can not work through their differences and eventually reconcile, but it’s uncommong for couples who grow apart to find a way to grow together again.