the horse can throw his head up and down, scratch his noise. No one is certain, most people say it is the sun and some people say it's allergies. My horse had headshaking syndrome but it has gone away. So no one exactly knows.
The headshaking manifests itself by violent uncontrolled head movements, the horse scratches / rubs the tip of the nose on any surface, often on its anterior or a barrier and even on its rider. The horse does not do it on purpose, unfortunately it is a true syndrome that the horse does not control absolutely
The causes are multiple, headshaking being a symptom, it is only the manifestation of the pain of the horse, so many pathologies can lead it ... For example, a simple sensitivity to UV can make appear the headshaking syndrom.