my 5 year old QH gelding used to do this too. How old is she. When my horse did it he was in his "testing" me phase and it was like its almost like a miniture tantrum. Or like hes bored and getting impatient. What i would do is everytime she paws at the ground i would back her up and make her wait. If she paws again, do the same thing. Repeat it until she stands without pawing and then continue on with what you were doing. It might take some time, but it will be worth it when it works.
Hope this helps, it did for me!