A dog's diarrhea can have many causes, including eating something they shouldn't (dietary indiscretion), ingesting toxins, foreign objects, sudden food changes, internal parasites, infections (bacterial or viral), allergies, inflammation such as inflammatory bowel disease, pancreatitis, side effects from medications, organ dysfunction, stress, or even cancer.
Common triggers are eating garbage or spoiled food, anxiety or stressful changes, infections like parvovirus, and parasites such as giardia. Sudden diet changes or food intolerance can also upset a dog's digestive system, causing diarrhea.
If the diarrhea is persistent, severe, or accompanied by other symptoms like lethargy, vomiting, or blood in the stool, veterinary care is advised to diagnose and treat the underlying cause.