This is so common it's not funny WubbaMom. Don't panic. As stated by others, it is probably a combination of the reasons listed by everyone. One of the hardest things there is to teach to a young pitcher that's made of rubber is proper form and execution of the pitch motion. Young ladies starting out find so many ways to make poor form work, simply because they can at that age. As they get older, the body begins to take shape, and girls need the form correct because they are pushing to keep up with others their own age they see as "Superpitchers." As stated, she'll need to work on staying open longer to let her arm clear her hips before finishing whatever pitch she is throwing. I am sure your pitching instructor has shown her drills to make this happen. Just think about it and it'll come to you. When that newly forming curve/hip begins shutting/closing too soon, they'll scream bloody murder till they finally fix it. It is almost comical as hinted above. It's the "Doctor Doctor, it hurts when I do this. Well, don't do it," routine.
All joking aside; I teach a drill we fondly call the "Flamingo" drill. While on the pitching rubber, have her stand on her push off leg and pick her lead out leg off the ground and position her foot behind the push off leg in the area behind her calf. This will have her standing on one foot like a flamingo. She'll need to be turned into a position that will allow a proper full arm spin of her pitch. Now, have her go ahead and deliver a pitch to the catcher and land on her lead off foot just as she would during a normal pitch, but finish in the opposite position with her lead out foot down and her push off foot behind the calf. She starts on 1 leg and finishes on 1 leg. This helps keep the hips in place while promoting balance and control. It's great for getting a pitcher doing just as you described to realize the need to keep her form true. With some practice they can get pretty good at this and it definately helps them remember to "keep the door open" till the arm clears the hips. With practice, the balance improves and they'll do this without ever putting the other foot down at any time.
I can't take credit for this drill. I was taught this by Cherrie Kempf and found it to be very effective for this sort of problem.