Why Your Horse Is Tossing Its Head

so try to ride when insects are less prevalent or ride indoors. For some horses the head shaking problem might not go away completely until cooler weather when the bugs are gone.
Saddle fit An improperly fitted saddle that is pinching your horses back can cause a number of unwanted behaviors including head tossing. Saddle fit is a very common reason for bad behavior. Check your saddle and how it fits your horse periodically as the fit of your saddle can change over time. 
Improper rein aids Often horses toss their heads in response to the rider mishandling the reins. The first thing a rider needs to learn is how to have light sympathetic hands that follow the movement of the horse while still maintaining enough contact to control the horse.

This is probably the most common reason for head tossing and the solution is to take a few lessons with an instructor who can help the rider learn to have a soft dynamic connection with the horses mouth. Heavy hands in conjunction with a harsh bit can cause some horses to throw their heads violently. Some horses may have to be reschooled so they trust the rider not to hurt their mouths.
Solutions vs. Masks
If you are tempted to solve the problem by using a harsher bit tiedown or martingale consider the possibility that you might only mask the problem and not solve it. Always view any unwanted behavior as your horse reacting to what you or someone before you has done to it. When a horse tosses its head it is trying to tell you something. This is how it is speaking to you. Its always more important to be a horse listener than a horse whisperer