Interestingly enough, a new client has been having issues with her horse throwing her head at the canter. I've not taught this pair before, but when she started mentioning wither soreness, I immediately thought "saddle." The horse is very wide through the wither and shoulder and although the saddle appeared to fit fine and was stable on the back, there was no room for shoulder movement. I gave her Liz's jumping saddle to try and voila! no head tossing.
Please, everyone remember to check the fit of your saddles often. You might feel just swell, but maybe your horse does not. Look for bumps, they are a sign of trouble. If you see hair messed up under the saddle area after you've ridden, consider saddle fit issues. Check the "dirt" marks on your saddle pads for consistent patterns. If you're unsure...ask a reputable saddle fitter to look at your tack. Your horse will love you for it!