Why dog won't stay in one yard?

Can you please tell us more?
Do you need help training your dog, or dealing with a neighbor?
I have never had a dog stay in my yard, despite knowing where the boundaries were.
It's part of their territorial aggression, if they are aggressive dogs (not all are). If the threat across the line is bigger and stronger than they are, they will stand right on the edge and bark and then go back to the house.