Hmmm. Good question. From my perspective, he's standing behind you because that's what he's supposed to do, in like certain situations (ladies first). But I doubt its anything to think negatively about.
Let's analyze. He could be:
*standing behind you, so he can be supportive, put you first and foremost. Respect.
*that's something he thinks makes you feel comfortable.
*(joke) he's checking out the goods in the rear. Lol. (sorry. Had to. )
---or he could:
*be uncomfortable standing next to you; does he not walk next to you when you walk down the street?
*not want to stand next to you, because he doesn't really want to show he's with you in a crowd of people. (I've seen this before. It's a classic "I don't like pda. "
*(joke) you smell funny. Lol
I gave you two positive, two negative and I think you should take those, put them in your own words and ask him. It's good to be honest with one another, and obviously it makes you feel a certain way, so ask him. Of course he may be defensive about it, so make sure you don't bring it up while it's happening.
He doesn't want other people to think he's dating you. Physical distance = emotional distance.