Hello and thanks for researching this very important question!
There are a few possibilities for what happened to this dear pet at this age:
1) Vestibular disruption...there are some structures deep in the ears that help the brain maintain balance. A number of things can cause a disturbance of the "vestibular" system in the brain, which is responsible for balance, among other things. These conditions include: ear infection, cancer, OR contamination of the brain with waste products usually handled by healthy kidneys and liver.
The chance of more severe problems increases with increased age of your dog.
Symptoms of vestibular disturbance include rapid eye movement from side-to-side, inability to stand or walk due to dizziness, nausea and/or vomit (with refusal or inability to eat), spastic head movements, and a tilt in the orientation of the head. Affected dogs prefer to lie on one side of the body only.
There is such a thing as "Idiopathic Vestibular Syndrome"...(idiopathic--pathology which doesn't make sense)...which does not have a cause that we can find. This is pretty common in middle-aged and older dogs. The most severe symptoms fade (or become adjusted to!) over the course of a few weeks, and the most prominent thing we see is the head-tilt...looks weird, but is not a serious problem long-term.
2) Seizure...Some seizure disorders have a cause, such as previous illness (diabetes, kidney or liver failure, hypoglycemia), toxins, tumors or physical injury (like being hit by a car) that has damaged the brain. Some seizure disorders are what we call "Idiopathic" (sickness without a known cause). Sometimes a dog will have one seizure, and never another. Most dogs with simple epilepsy can be well treated with drugs.
Seizures can be brief (sometimes there is not even loss of consciousness), or prolonged. They can be mild in their appearance or physically violent. The more vigorous and the more prolonged a seizure is, the more dangerous it is to the brain.
An older pet experiencing a violent seizure, or repeated seizures, is more likely to be harboring a serious health problem, such as cancer, brain hemorrhage, or organ failure.
Diagnosis can be as simple as running some blood tests, or as complicated as undergoing an MRI to find a tumor or other brain-threatening condition.
***The first step to finding out what happened and if it will continue to be a problem is to have this little one examined by your regular veterinarian.***