I have a 21 year old Arabian gelding who slings his head all the time. It really makes for an uncomfortable ride. He slings really bad while riding English especially because that is when I focus more on his headset then in western or bareback riding.
We think that it is caused by pain in his back, teeth(this is the least likely) or his eyes. We also think he may have head shaking syndrome. We are going to see about getting a chiropractor to adjust his back/neck and see if that helps at all but even in the past he has done it. His eyes also might be bothering him. They are strange. They look like like the brown color is gloopy and running into his pupils. He has strange gloopy stuff in it. Nobody really knows why(even the vet) but we think that whatever it is might make his eyes sensitive to sunlight(though that doesn't explain why he slings his head in the barn or the indoor arena.
I can get pictures and I have a video of his headslinging in the last show I went to(he was really bad then). It's just really annoying and I'd like to get it stopped.