I guess she just likes to stretch!😂
Maybe you haven't had a long enough warm up.
It really just might be your horse's nature but it also might be because the reins are a bit too loose or too tight. It may also be because of pain so she wants to stretch her neck. Watch her behavior and if you thinks its serious go visit a vet. If it's just her nature than slightly kick her sides until she brings her head up. Usually, if you make you horse go a bit faster they will also bring their head up also. Hope this helped!
The first thing you should do it to have a medical check up to be sure this behavior doesn't come from a back or neck pain.
If it is not coming from some pain, horses often do that to stretch their back.
At the beginning of the session, you should let him to that so he warms up slowly and in the way he wants.
then, when you feel that he is more comfortable, ask him gently to pull his head up and ask him for more impulsion and hind legs engagement.
At the end of the session, you can let him hold his head low to stretch his muscles.
Good luck :)