As annoying as it may be for some owners, some cats simply enjoy digging and playing in their litter boxes. Most of the time, this is normal behavior.
But sometimes scratching in the litter box becomes excessive. Maybe something is frustrating her? Does your cat have enough litter material to fulfill her instinctual urge to cover up waste? Is her box big enough so she can perform her normal elimination behavior? Is her box kept clean? Is her box in a calm and private location?
If the answer to the questions above is yes for all questions, then maybe your cat does not like her litter. A wide range of cat litters are available today, and cats react differently to different products. So try another type of litter and see how the behavior is impacted.
If you have tried everything and the excessive scratching persists, consult your veterinarian as this behavior may be caused by a physical problem.