Now that I know what causes it, what can I do to stop it? First, take your dog to the vet to make sure theres no medical reason for the submissive urination such as diabetes or a bladder infection. If it's not a medical issue, then here are a few tips to help stop this behavior.
Whenever you greet your dog, especially after a long period of absence, keep it calm and mellow, don't get him all excited.
Do not punish or harshly correct your dog for this behavior. He's not doing this on purpose. If you catch him in the act, simply interrupt him and praise him when he stops - but don't punish him.
Limit his intake of water to help him control his bladder especially whenever a situation is coming up which would normally result in urination.
Common house training problem #2: Scent marking
Scent marking occurs whenever a dog "marks: his or her territory with urine. This problem is based on issues of dominance and territoriality. Technically this is not a house training problem.
Here are a few tips on deciding if your dog really is scent marking or not: your dog is male, unneutered, and at least five or six months old; only small amounts of urine are produced and tends to be directed against walls or doors; you live in a multi-dog household where there is conflict between two or more of the dogs.
What can I do about the problem?