Like the title says, I noticed one day (first time he's done this) that he wiggled his mouth to try to grab one of the reins while we were riding.
I haven't noticed him doing this again, but I was just curious about what you do about it if it does persist? When I noticed him doing it, I wiggled the rein to get it away from him lol.
I work with two horses that like to grab the sides of the shanks with their mouths. Its REALLY annoying. The best solution I found was to take the reins and whack them in the side of the face. Not hard, just enough for them to associate grabbing the bit with something unpleasant. One quit doing it totally, the other will do it on occassion. She is typically anxious and also shows other anxiety based behaviors like the ever lovely "flamingo stance". She has also been doing it MUCH longer then the other horse so nip it in the bud FAST. ;)
Along with that look at some of the stuff your doing... WHY does he want to grab that rein? Bored, to hands-y, something pinching?
My driving horse did this all the time! Normally when we were at a red light and I let the reins relax, he'd pick it up and when I got him going again I'd quickly realize he had the reins - not me ;)
I found a sharp tug and a slap with the rein works. He also did this ALL the time while we were standing and waiting for a ride - I'm sure it was boredom.
You could look into bits with rollers to eliminate that potential cause.
Doesnt sound like he does it all the time. It does sound like your horse is trying to tell you something, boredom, slack rein or just something to do. He might be one of those horses that likes to be doing something different or challenged in some way. Maybe a different trail, open a gate, ride over or around some obstacles just do some change-ups to your rides. Try and observe what is happening when he does it again. Then correct the bad behavior but you might have some insight of why he is doing it.
Scrat does this when we are walking, or standing. Also when he is nervous he starts trying to grab the shank or the reins. I just wiggle it out of his mouth and go on...I usually catch him before he succeeds and after a few tries with failure stops. I don't worry about it to much...
Ruger, oh lordy. If we're standing still he will reach around and chew on the reins. The first time I put a shank on him (Tiny tiny shank jr. Cowhorse) he grabbed the shank and flat INHALED the rein, and I had to fish it out. He reminds me of one of those clowns who eats handkerchiefs, ya know?
I'd just smack him with the end of the rein or a crop or something. I mean not like abuse to the face, but deterrence.
We used to have a pony that had this issue. She constantly would try to get the bit shank into her mouth, and the reins to follow. With her, we used an "out of the box" method to get her to stop. She HATED having her ears touched while riding. So every single time she would start to play with the bit, whoever was riding her would immediately reach up and GRAB her ear. (She was a pony so that neck was nice and short to reach easily.) She caught on pretty quick that her ear would get grabbed if she went for the bit. That took care of that!!
But normally, I would probably treat this the same way that I would treat a horse that dives for some grass when you are out trail riding: One hard smack on their upper neck with the end of the reins and a loud "NO!". That gets your point across.