Dreamcatcher basically covered it.
To add, I've known a couple horses that would paw, one did it just because (bored, I guess), and the other did it because he was nervous.
The "just because" one never really resolved but he had a lot of occasions to get bored (rental string horse, tied up all day, etc) so I just basically let him do it since I couldn't set him up for success.
The other guy, once he settled in, the pawing completely resolved itself. He'd still paw if a mare was in heat (studdy gelding, haha) or if he was spooked by something while tied and had no other outlet but as a day to day thing, he didn't paw after he got comfortable.
I did find that if he was punished for pawing (some other people at the barn liked to punish him for pawing) he would start doing it out of "spite". Like, he'd paw the air while they were looking, then paw the ground - making as much noise as possible - while their backs were turned, and as soon as they turned back around to punish him, he'd immediately go back to pawing the air while looking at them with ridiculously innocent eyes like he had been pawing the air the whole time.
He was SUCH a cheeky pony.
I finally got them to stop punishing him and we started ignoring him, like Dreamcatcher suggested, and as he settled in, the pawing totally stopped. He'd still paw the air in anticipation of being fed but yknow, haha.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.