Very normal behavior when coping with anxiety or just bored. Give her something more appropriate when you catch her in the act. Let her chew on gum, laughy taffy or something else. You can buy these chew straws from Occupational Therapists. They are in the shape of a letter T. It sends sensory input to the brain and is calming. It's just like an adult who taps their foot, twirls their hair, cracks their knuckles, etc. Here's a link to buy them online. Click on "oral Motor"" then "Designed to Chew." Hopefully by focusing her attention on something else, she'll save her beautiful nails. :) Good luck.