it’s easier for them to do it at a standstill than walk - and they can’t do it at the trot… if you know it is coming kick her on - tips for your DD - grab mane and go into a half-seat.
mine used to do it in our first saddle i had fitted to him - a kieffer. for various other reasons it appeared to not be a good fit for him (he started acting girthy, headtossed when the saddle was picked up, started tensing U.S) and it disappeared entirely when i switched to a different saddle.