If he is acting normal in every other way eating drinking foraging interacting with his brother maybe it is just a quirt with him. I just can't imagine what it could be and since he only does it every so often, Some things are just baffling. Oh does he do the tail tucking when walking or just...
It's very strange could other drakes be trying to dominate him just wondering if he does this when around other drakes? You only have this drake and another?