It's not difficult to understand - now, I think. How it started, is another matter. Of course she is nervous - because she makes mistakes; the more bad SPs she does, the more she will EXPECT to make mistakes, and the more nervous she'll be, and the more mistakes she will make... It's a vicious circle, really. She is more relaxed in the LPs - because she knows she can do them - because she HAS done them.
How to FIX the issue, I think is indeed for a sports psychologist to sort out. Practically, of course, what must happen is for her to do clean SPs - the more clean SPs she does, the more she will know, in her heart & soul, that she CAN do them. But first the vicious circle has to be broken. And that is - to put it mildly - the tricky part.