It could be he needs his teeth done. Or he could be in pain. A chiropractic visit might be in order. Does the tack fit? Does the bit fit? How many wrinkles did they have in his mouth? A lot of times a horse will mouth the bit if they have wrinkles. I just recently went out to see a friend's horse and she said he chomps on the bit a lot. She had 3 wrinkles in his mouth (with just a snaffle) and I lowered it A LOT. Within about 15 seconds he was standing quietly.
Horses will sometimes do this as displaced behavior. If the rider is holding the horse back or the horse is getting worked up and becoming emotional you could see this behavior. Either way it's the rider's responsibility to help the horse through the situation, not add fuel to the fire.
Some horses don't like a lot of tongue and bar contact. Our mustang HATES snaffles. He puts his tongue over the bit and just acts antsy. We put him in a Myler bit with maximum tongue relief and he was great! So this might be the case with this horse. Horses like this will usually exhibit behaviors like chomping on the bit, opening their mouths, head tossing, tongue over the bit, yanking the reins from the rider, etc. So a new bit might be in order.