I was always told it was frustration about not moving whilst eating
Out of interest, do the horses that paw when eating also do it at other times?
Turned out on fresh grass?
When they are tied up?
Or when they are made to stand still when ridden?
Or in a trailer/lorry?
Is pawing (either when eating or restrained) prevalent through all ages/temperaments/types? Hotter/laidback/greedy/nervous/young/old ?
Logically, pawing when movement is restricted (tied, or made to stand), would indicate impatience/temper/stress (are those three one and the same in reality ) at not being able to move as they wish, or be with others.
Mine just lifts his left leg up while earing his hard feed. No pawing, just a dangling leg.