Why can dogs suddenly express milk?

The ability to express milk from dog nipples is a symptom of false pregnancy. A female dog with false pregnancy will have a swollen abdomen, mammary glands filled with milk, and significant changes in mental state. This disease is caused by hormonal levels in the female dog, and the symptoms usually appear in dogs that are in estrus or are not pregnant. You can increase the dog's amount of exercise and do not give it toys or similar items, because the dog will take care of the toys as its own children.
In severe cases of false pregnancy, veterinarians will generally recommend that the female dog receive hormonal treatment. Under normal circumstances, female dogs with mild illness should transition through this period on their own.