jackson head jerking ?
head jerking (not to be confused with trembling or shaking) is something that male xanths sometimes do, particularly if they feel threatened or that their territory is being invaded, especially by other chameleons (or even predators or other lizards). i dont know your location, but if you were in hawaii, where geckos are everywhere (and would probably be crawling on the outside of his cage), that might encourage a lot of head jerking. xanths overheat rather easily. if he is kept outside, you need to monitor your basking temps (especially if outside ambient temps are above 80*) and provide easy access to cool shaded areas jmo