If the dog is a puppy it is an indication that the dog is teething In older dog there are a couple medical things that need to be looked into.
Mine tend to be snout rubbers when allergy season kick in. Other causes include lip fold pyderma in which salivia and deberis gets caught in lip fold on the lower jaw, this is general accompanied by some pretty rotten breath, In most cases cleaning the area a couple times a day is enough in other like Mariah surgical removal of the fold is required. Paridontial disease, something caught in the mouth and/or between the teeth can cause this as well.
Mine tend to be snout rubbers when allergy season kick in. Other causes include lip fold pyderma in which salivia and deberis gets caught in lip fold