She in all likiehood does it because it's a habit. She might be bored. And because she's a lab. They have a reputation for eating anything they can--edible or not!

Preacher used to eat rocks when he was younger. He only did it when he was bored. If he was left alone for too long, he'd find a rock to eat. His last owner had to have surgery done 3 times to remove rocks from Preacher's intestines. He's outgrown it as he's aged.

Watch her for signs she's blocked up. Eating rocks is also extremely bad for their teeth so check them too for cracks or broken teeth.