So I would say that feeling pressured is the most common reason that you will see a man become distant. The more you His Secret Obsession pressure him, question him, try to gauge him, or make him feel negative emotions like guilt and shame, the more likely he is going to be to distance himself.
Another reason that you might see him becoming distant is that he is under the influence of friends or family who do not share your cause. Often, his single friends will try to pull him into their lives or his separated or divorced friends will tell him how much better their lives are now that they are single.
Sometimes, he backs up a little because he realizes that he's not yet experienced those things or explored those feelings that he meant to when he began the separation. In short, he might feel that he became distracted while things were going so well between you, but now he needs to see things through.
How To Respond When You Don't Understand His Distance: I would caution you to not overreact. I know that this hurts. And I know that it is hard not to assume the worst. But if you come at him with all sorts of accusations or pleas for assurance, you just might make this worse. Because there is a decent chance that this is just a passing thing that will fade to memory once you bide your time.
So to the extent that you can, take him at face value until he gives you a concrete reason to stop doing that. Continue to do what was working before. Remain playful. Keep flirting with him. See where that strategy continues to take you. Do not panic. Do not apply more pressure. If you do these things and you meet some resistance, then you may wish to back off and see if that will entice him to move toward you.
Sometimes, your own silence will inspire his curiosity. I would caution you, however, not to take this to extremes. Don't out and out ignore him, pretend that you don't care, or make him think that you are seeing someone else. You want to behave in a way that you can be proud of. And you don't want to do anything that can come back to haunt you later.