Infact, i've seen something similar to this. There was an Arab back at the facility I used to board at before I got my own land... and he would open his mouth and flip his head everywhere, tounge lolling all over the place. He would also tense his lips and make fart noises o.o; seriously. The first time I heard it I couldnt believe it!
That's what happens to horses that spend their days cooped up in stalls, they develop really weird habits/vices (usually the're bored, stressed or have too much energy). What that horse is doing may look funny, but that behavior is on the same lines as stallwalking, wall-kicking, and even weaving.