Salisbury steak (and ground meat mixtures in general) will fall apart if it doesn't have enough binding ingredients in it, or if it has too much. It is a balance. When making Salisbury steak, use ground meat that is 80-89% lean. If the ground meat is leaner than that, you will need additional binders to hold it together. Do this by adding an extra egg and another 2 tablespoons of bread crumbs to the ground meat mixture.