There are many reasons that cause a dog's mouth to tremble, such as: 1. When the weather gets hot or cold, the dog's mouth will be uncontrollable, causing the mouth to tremble; 2. Eating food that the dog cannot eat by mistake causes the food to tremble. Poisoning causes the mouth to tremble; 3. The dog is unwell, accompanied by loss of appetite, lack of energy, etc., causing the mouth to tremble; 4. The dog's lack of calcium or nutrition leads to nervousness, causing the mouth to tremble.