Why do horses do head tossing when sweaty?

Horses head toss when reins are too tight, when they are bored, or when they think they are being overworked. They are basically just being stubborn. A good way to stop it is loosen the reins, stop, and give them a minute to rest. Then go right back to working them - otherwise, they'll think the head tossing gets you off, and they'll make a habit of it.