Rabbit Litter Tray

[nic3c]

My rabbit used to use her litter tray and then started digging the litter out of it and tipping it upside down and then started using a different corner of her cage. We bought another litter tray to save cleaning her wee up all the time and she now uses both but still likes to dig the litter out and make a mess. Why is she doing this and can we train her to go back to using one tray?