Why do dogs pass out?

My first thought would be temporary circulatory collapse, which may be caused by a bleeding lesion inside the dog. Certain breeds of dogs, including Golden Retrievers and German Shepherd Dogs, seem to be at increased risk of a certain type of cancer called hemangiOSarcoma. This is a tumor of the blood vessels in which new, very weak blood vessels build off of existing blood vessels. Under normal blood pressure, the tumor vessels rupture, causing internal bleeding; if the original tumor is benign and is removed, the dog can be successfully cured of the cancer.

Other causes would include gastric ulcers, epilepsy, congestive heart failure or other cardiovascular or neurologic conditions. Regardless of the cause, this dog needs to be seen by a veterinarian soon to determine what is causing the collapse episodes.