Living here in the ranching country of western U.S. provides a lot of opportunities to understand our working horses and their oddities. You say you've placed a guard on the horse for pasturing... I'm assuming you're taking it off once he/she is returned to the stabling area, which is where the horse is eating the straw, no?
You've probably caused the problem without realizing it. You've noticed, I'm sure, the horse, sometime in the past, occasionally knawing wooden structures in the barn or fencing. If so, you've discovered that horses like to chew. Since you've prohibited him from satisfying that urge by use of the guard while in the pasture, he's most likely simply chewing the straw to, well.... chew the straw.
Like the others have suggested, it's not a bad idea to have a Vet look at your horse, but absent any sickness symptoms, the Vet is probably going to come up empty handed. We see a lot of chewing by our horses, but that's just what horses do. Are you exercising the horse adequately? If not, he could just be bored which results in additional chewing.... Best of luck!