Head throwing exracer

[Amy56]

My new horse is an exracer and she is perfect in the school and tries her hardest (has a bad rein) but I know that's typical of an ex racer , but she still tries.
However out hacking after about 15 mins she starts to throw head head up and down if you let her have the reins on the buckle. She only does it in walk and isn't joggy or strong with it. And it's litrally only hacking she does it. You can school her for as long as you like and she collects up nicely and does throw her head at all.
I'm wondering if its a thing she has picked up as a habit

 

[Jessey]

I could be a habit, perhaps boredom, or anticipation (in race yards they often 'hack' to the gallops for training). If she only does it on a loose rein perhaps just maintain a loose contact to stop it becoming an ingrained habit.