My dog would air snap virtually everytime I had her back up all through her puppyhood. As she's gotten older she's stopped mostly. I always took it to be playful excitement, but I can see where it's a little backtalk-y too (I know, new word of my own making...don't see it catching on any time soon LOL), so perhaps a little defensive. Either way it diminished with maturity on it's own, and was never threatening or aggressive in any way. Like barks and growls and nose pokes, it's communication, so I would never correct it!
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain