Dogs urinate for reasons other than to relieve themselves. Two of the most common are marking of territory and showing submission. Submissive urinating is intended to communicate exactly that, submission. The dog is saying, in effect, that he acknowledges that the other dog or person is the boss. In some cases the message is that he or she is actually afraid of the other.
The dog usually displays certain submissive body language such as cringing or crouching with tail tucked and ears pinned back or rolling on his back. Although it can happen with any dog of any age it's more common in females and young dogs or puppies. Again, this behavior is a hold over from life in a pack where social status was very important. Some other reasons dogs do this are over correction, punishment or abuse, but that's not always the case.
Some dogs are just naturally very submissive and are hyper-sensitive to something like a sudden movement or a deep male voice. Over excitement and perceived danger can cause these dogs to lose control of their bladder. A stressful event like the introduction of a stranger to the group can cause a similar reaction. Often, the excitement of greeting the owner, say after the work day, can cause submissive urination incidents.
This behavior has its place in the natural life of a pack animal but can be a real problem with a pet. There are several ways to cut down on the accidents, however.
The key here is maintaining a calm and reassuring manner and being careful to convey your intentions to the dog. When you arrive home form work or are greeting your dog after a separation approach with your hands in plain sight and reach to pat him in an underhanded manner that allows him to see your hand, rather than touching him on the back or top of the head.
Don't look directly into his eyes as that is construed by dogs to be a dominating or threatening behavior. Speak quietly and reassuringly and keep greetings low key to cut down on excitement. At first, you may want to arrange your meeting place when you get home from work, for example, to be outside or protect the inside area.
Ask your dog to join you outside if possible when you open the door. Often, young dogs will outgrow this problem as they gain confidence about themselves. The all- important thing to remember is that any form of scolding or punishment will only make the situation worse.
Don't raise your voice or speak in a threatening way. Remember that submissive urination isn't a wholly controllable action. Have patience and do all you can to help your dog feel safe and comfortable so he can overcome this behavior.