It all depends on where she is doing it...
Is this over the floor? Into a pit with a wedge mat?
Some coaches sorta weed out gymnasts if there are a lot of them.. it's sad, but true.. this may be one method this coach uses. However, without seeing it, I cannot say for certain. Do any other gymnasts have this problem?
Have you talked to the coach about it? Expressed your concerns? If you have, what was your response?
If youre asking about "why" it's happening, it's one of a few things - one, could be arm strength. Again, it could be her jump angle - too low, and she will have to bend to get through. Last, she could be wipping her hips to make the flip-flop (back handspring). It's hard to tell without seeing it...
What can you do to help? How are her back walk overs? Does she play with them at home? That can help with the arm strength.
If it's not the arms, help her feel what it's like to push the floor at 45*. Basically, jump back and away from the floor like fred flintstone stopping his car.
Wippy hips... well.. that takes some more effort, and some coaching. Could be positioning, or a mixture of problems, like arm swing, throwing her head too early... again.. it's hard to say without seeing..
Im sorry I cant give you much more help... if you can post a video we might be able to help more.. though I really really encourage you to talk to her coach. Sometimes, if you appraoch a coach with a genuine concern, away from the gymnasts (and your daughter... and preferrably with another coach in ear shot), asking nicely will get you a good result.
Let us know how it goes.
Ryan