Unknown. When nerve cell in brain fires electrical discharges abnormally, these discharges affect other nerve cells. Our brain has protective mechanism to avoid spread of those discharges. One cause is failure of the mechanism, why one nerve cell fires abnormally is stll not completely answered. Many researches are going on and more and more new informations are coming up but still no final answer.
Specific triggers. The brain is a complex array of electrical circuits that are highly regulated and controlled in the usual setting. For some patients this regulation can lose control and go into an electrical storm resulting in a seizure. Anyone can have a seizure if this regulation breaks down. Certain drugs, toxins, alterations is blood chemistries, excessive heat, stress, and sensory triggers may do it.