I've tried to resist doing this but it happens quite automatically. If I don't fully concentrate on my mouth at that moment, it goes right back to eating air. And it doesn't just happen to me. When other people feed Baby J, they do the same thing regardless of whether or not they have kids of their own. It's kind of funny to watch. We're feeding her but taking imaginary bites of our own as we do.
I'm guessing that this sort of thing happens anytime people feed babies. I doubt it's a phenomenon localized solely around my daughter. It must be some universal parenting thinggy we all do.
Next time you have the occasion to watch someone feed a baby, watch their mouth and see if the try to eat phantom food.