Could be many reasons, who knows why they do stupied crazy things !! Just to make us mad !!! maybe she's bored, she could just be checking the egg out, she might want to eat it !!
Is she a new layer ? How long have you had her ?
I had a hen that was doing this during the summer, I put a few fake golf balls in the nest where she laid and she stoped doing it right away. Guess she got bored of not being able to put a hole in them. I would give it a try. Whatever you do try to break the habit fast, once a chicken starts eating there eggs it's hard to stop...