My mare does it sometimes, for 3-4 minutes, usually while nothing is going on. I've concluded she's bored and wants something to play with. She doesn't do it when we're busy, and it often comes when there is slack in the reins and nothing is happening...just straight down a path. It is about once/ride, any bit I've ever tried, including after getting her teeth done. The rest of her body is relaxed when she does it. Don't know if that the same or not.