Nipping can be a sign of frustration, especially in high-drive, octane-fuelled dogs.  They want to know what to do next, and they want to know it now! I see their thought processes as ‘Why so slow, person? WHY SO SLOW? If I nip you, will you think faster?’  In order to head off this kind of nipping, don’t let your dog become so frustrated with you. Either give the directions quickly, or keep the dogs arousal lower. (I realize that slowing a dog down is not usually suggested, but at a certain point revving up is counter-productive, at least until the handler can work the dog when it is fully excited.)

Another reason for nipping is the dog simply never learned that nipping is unacceptable social behavior.  This can be easier to deal with than excitement nipping – or harder, it very much depends on your dog.  Some trainers have success when they remove the dog from the course at the first graze of teeth.  Also, redirect the dog’s energies to an appropriate item, such as a tug.