I have heard that the muscles of the neck (or lack there of!) can make the mane swap sides. My guy, in constant PROPER work, wears his on the right. When he's being a pampered pasture pet, it falls to the left. Oddly enough, I've never seen it in an "in between" stage, and I've never actually noticed it happen overnight or anything. Just one day I'm like "Hmm, thought his mane was on the other side a few days ago!"
I've had some with manes that I could put on whatever side I wanted and it would stay. Where the horse wore it depended on which side of the bed he got up on (or how vigorously he shook when he got up). My suspicion is that the texture of the hair is what makes it easier or harder for the mane to be able to go either way.
I've had the mane switch sides on a horse I rode a lot, but it didn't happen over night.
Here's what happened: the mane started out on the right. I've got a problem in my pelvis (suspect torn SI ligaments from 20 years ago). I don't think I sit evenly, judging by the sweat pattern under the saddle, no matter what saddle I use and it happens on every horse.
Over time, the mane split down the middle. Then it went to the left.
I was training this horse for distance competition and was riding close to 20 miles a week for conditioning. He came up 3-legged lame from a nasty leg abcess and was on stall rest for almost 5 weeks. His mane reverted to the right in that time.
When he was sound again, as I gradually built up riding time, his mane started to split down the middle again as though it had a part. For other reasons I stopped riding the horse. Mane reverted to the right.
I'm thinking that I made him carry himself differently through my unevenness, and this affected his mane. Just a guess.