Syncope is most commonly caused by heart disease, but,
tracheal collapse is also a strong possibility. These are the most likely causes of your dog's problem since your dog routinely coughs prior to the episode. Cough is common in both tracheal collapse and heart disease.
Seizures can be caused by brain tumors, chemical or toxin exposure, infectious agents that affect the brain, head trauma, and others. There is also idiopathic seizures, but these are usually seen in younger dogs.