Horse Veterinary Questions? Ask an Equine Vet.
This is a Flehmen response. This is a behavior that horses perform to assess the scent of other horses. By doing this behavior, pheromones are delivered to the vermonasal organ which is an organ that helps the horse identify the physiologic status of other horses. For example a stallion (and sometimes geldings) will smell the urine of mares to determine if they are in season and likely to be fertile. Mares will do this to identify their baby, horses of both sexes do this when investigating a new territory or an area that has been marked by another horse.
It is a normal behavior and nothing to be concerned about. It tells you that your horse is trying to get information on the horses around him.
Here is a link to an article that explains it in great detail.
Does this answer your question?
The middle video (one where he is in the stall with shavings) looks most like a flehmen response. The other videos appear to be a horse that is lip smacking because he is bored. Another possible cause of lip smacking is abdominal pain from GI ulcers, colic, impaction, entrapment, etc. If he is otherwise healthy and eating well, defecating normally and then ulcers or boredom would be my first two differentials. Has he been examined by a veterinarian? How long has he done this behavior?
Is your horse eating normally and acting normally aside form the lip smacking behavior? Does he have any other health issues that may be causing him discomfort?
What is his housing situation?