Why is my turtle still trying to escape?

Okay, so my res turtle is pregnant. She kept trying to get out of her other tank (I figured she was trying to find some other place to lay her eggs, the other two aren't trying to escape) and anyway i put her in her egg laying tank which has lots of dirt and moss in it, and after about an hour, she's trying to escape from it! she has water, its pretty good size (20 gallons) and she has food. Why possibly is she trying to escape now?