Truly, I think your horse is probably just relaxed!
I pony a polo mare that does this as we trot. As many times as I've told her she could damage her teeth, she doesn't listen! This mare lets her bottom lip droop sometimes at the walk. So it swings from side to side, making her look a bit goofy. You'd never guess what a competitor she is during games from watching her while ponied. Lol