He could be reacting from your contact. Is it strong or light, try changing it to what it is not and see if that helps. He could be reacting to too strong of a contact or annoyed that the bit is too mobile in his mouth.
My horse did that for a while too. We thought it might of been because he had a sore tendon but he still continued to do it after it was fixed so we’ve changed bits around. Now since we found the right one, he isn’t doing it as bad. It could just be a habit which we think is what my horse is getting into. My instructor said that when he does that to just send him forward as soon as you feel him raising his head to toss it.
Well head tossing really can be anything like sensitivity to bugs, dust, rain, noseband or bridle fit in general, bit, teeth and health problems, or just your hands and maybe him being annoyed by something you do. Soooo basically try everything 😂😂