How old are your horses? If they are young'uns, they will sometime pick this behavior up =)
I wouldn't blame lack of mineral, but good on ya for thinking about that (a lot of horse owners don't understand how important the mineral licks are!)
Were the halters always where they are now? Or did you just start leaving them out there? Are they usually really lippy and licky when you pet them?
Skippy!, my first horse (and current like.. ruler of the 7, lol) Likes to snoot around in manure, occasionally nibbling a nugget.. he likes to lick the dust off of his pipe corral, chew lead ropes, eat wood, chew buckets, and lick rocks. My dad and were absolutely stupified. He had a mineral lick, and was getting a supplement bucket on top of that. We asked the vet when he came out to give vaccinations.. and he asked questions, all of which were options we had already done.. and he said something to the effect of "Well, I think your horse is very oral, and just enjoys feeling the world through his mouth". A very interesting point of view! Lol!
The one thing I would really advise you to be careful of, is make sure your lead ropes are securely tied to the fence/pipe corral in a safe way. Just incase one of them chews and pulls enough of it off to paw at it and get stuck, or entangled.
I hope that helps ya out some =)