While sometimes it doesn’t affect you, a horse tossing its head while you’re riding can throw both you and him off-balance. Katherine Blocksdorf of About.com suggests some reasons that your horse may be tossing his head when you try to ride him:
Insects
A very likely reason for your horse tossing his head is annoying insects buzzing around his ears. Consider some insect repellent and perhaps even ear covers if you see the bugs buzzing around.
Mouth
There are few things that can go wrong in this area. Always make sure that your horse has good dental health. If he doesn’t he could have sharp edges on his teeth where he bites into his cheeks or tongue. You’ve probably done that a few times yourself and no, it doesn’t feel good. If that is the case, be sure to consult an equine dentist.
Another problem in his mouth could be the bitting. Make sure that is comfortable by checking how it tight it is, where it’s positioned, etc.
Saddle/Reins
An improperly fitted saddle can cause a lot of problems. Along with back pain, the horse may also be prone to toss its head. Pulling a rein wrong can also cause the problem. You must control the horse with subtle movements. If he doesn’t like the way he’s getting handled there could be negative results.
Source: About.com