Why do horses flap their lips?

Some horses will deliberately flap their lips while being ridden, or constantly. Why do they do this?

There are several reasons. I knew one pony who flapped his lips when asked to do tedious or repetitive work - so I suspect he was doing it out of boredom. He didn't do it if the job was more interesting.

Some horses flap their lips out of anxiety - it's not uncommon for horses to flap their lips when learning a complex new maneuver.

Horses may also flap their lips after being given oral medication - perhaps a signal of disgust at the lousy taste.