Its something cats do in the wild, an instinct to hide food in case it gets eaten by another animal,but being a pet they don't need to cover it up but instinct is still strong,many cats do this if not most, its as if they are scooping up something that isn't there to bury the food,
then only after scraping all around the dish they are then satisfied, even though it isn't covered,but mentally to them it is well hidden and will still be there later safe,so her reason is hiding it for later, just as dogs hide a bone for the same reason,hiding it away from any possible animal that may come along,
so she is instinct driven and all natural behaviour,nothing to worry.
