There can be numerous possible causes pertaining to virtually any body organ system, but some of the more common are neoplasia (cancer), GI parasitism, liver or kidney disease, inflammatory bowel disease, food intolerance, diabetes, hypoadrenocorticism, chronic pancreatitis, bacterial or viral infection, partial obstruction of the intestines (either from a cancer, intussusception or foreign body eg. (vetbabble.com)
Even dogs suffering from partial obstructions that pass on their own need veterinary support to manage pain and ensure no complications arise. (reference.com)
The condition is caused by some type of partial obstruction in the dog's upper airway such as a foreign object such as a blade of grass, chronic infection or in rare cases, a nasal tumor. (dog-health-guide.org)