Why is my bunny digging holes?

I have a bunny she is about a year old now. And she just got an outdoor cage it has a run in it where she is out on grass and is caged in but she eat all the grass and is digging holes all over the place. im not sure why this is so if you know or think that you know why she is doing this please tell my. Oh and if you no what i can do to stop this i would like to know that to so please help me before she digs out of her cage. Thank You