I can't view the video either, but we do raise Silkies. The head twitch could be a number of things, but I'd start with new pin-feathers coming in on their crests. We see this often and even have two 4-month old chicks ourselves right now - not much older than yours...
Both of our birds have LOTS of feathers coming in on their heads right now, and they shake them periodically because it's somewhat itchy as the wax tube (that the feather unfurls from) doesn't always cooperate with the bird or the feather it's holding. They'll scratch at them and twitch their head because they can't reach the top of their head easily with either their beaks or their feet/nails.
I wouldn't worry about this too much right now. If you'd like to "help" them, you can try to release the feathers from the wax sleeve by gently grasping the wax that's around the feathers near the base of the feather with two fingernails and gently pulling away from the bird to release the wax. We do that sometimes when we see a bird shaking her head because of feathers coming in, but it usually works itself out in a couple of days with no "help"...
